Osisko Development Announces Optimized Feasibility Study FOR Permitted Cariboo GOLD Project with C$943 Million After-Tax NPV5% and 22.1% IRR at US$2,400/oz Base Case GOLD Price; at US$3,300/oz Spot GOLD C$2.1 Billion After-Tax NPV5% and 38.0% IRR

ODV NYSE TSXV News Release

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OSISKO DEVELOPMENT ANNOUNCES OPTIMIZED FEASIBILITY STUDY FOR

PERMITTED CARIBOO GOLD PROJECT WITH C$943 MILLION AFTER-TAX NPV5%

AND 22.1% IRR AT US$2,400/oz BASE CASE GOLD PRICE; AT US$3,300/oz SPOT

GOLD C$2.1 BILLION AFTER-TAX NPV5% AND 38.0% IRR

(All dollar amounts are expressed in CAD dollars, unless stated otherwise)

Montreal, Québec, April 28, 202 5 – Osisko Development Corp. (NYSE: ODV, TSXV: ODV)

("Osisko Development" or the "Company") is pleased to announce the results of a positive optimized

Feasibility Study ("2025 FS") for its permitted, 100%-owned Cariboo Gold Project ("Cariboo Gold" or

the "Project"), located in central British Columbia ("BC"), Canada. The 2025 FS was completed by BBA

Engineering Ltd. (" BBA") as lead independent consultant, and supported by other independent

engineering firms, in accordance with National Instrument 43-101 – Standards of Disclosure for Mineral

Projects ("NI 43-101"). The Company intends to file the technical report in respect of the 2025 FS (the

"Technical Report ") on SEDAR+ (www.sedarplus.ca) and on EDGAR (www.sec.gov) under Osisko

Development's issuer profile within 45 days of the date of this news release. The 2025 FS confirms

strong economics for a low-impact underground operation using mechanized bulk mining methods, with

attractive operating costs, manageable capital requirements, and well-positioned to benefit from

favorable macroeconomic and gold price trends . The process facilities have been designed to

accommodate potential future throughput expansions.

Sean Roosen , Founder, Chairman and CEO , commented, "The completion of this optimized

feasibility study represents a critical milestone for the Cariboo Gold Project, one of the few undeveloped,

permitted gold projects in a Tier -1 jurisdiction4. The results reaffirm our view that Cariboo is a high -

quality asset with robust returns and significant upside potential within the existing mine plan. Our

immediate focus remains on advancing project financing and further de-risking the project toward FID,

but, we believe additional work could support potential future production increases within the planned

mine footprint. Additionally, o ur extensive land position around the Project area offer s numerous

opportunities for new discoveries in this prolific gold belt. With today’s favorable gold price backdrop

HIGHLIGHTS1

 Robust returns with base case after-tax NPV5% of $943 million, unlevered after-tax IRR of 22.1%

and payback 2 of 2.8 years at $2,400/oz gold price assumption. Using spot gold price of

$3,300/oz, NPV5% improves to $2,066 million, IRR 38.0%, and payback2 of 1.6 years

 Average annual production of ~190,000 ounces of gold over a 10-year mine life (202,000 ounces

in the first 5 years) with first gold anticipated in H2 2027, assuming construction commences in

Q3 2025, subject to progress on ongoing project financing discussions

 Average TCC of US$9 47/oz and AISC of US$1,157/oz over the LOM , placing the Cariboo Gold

Project within the lower half of the global cost curve for gold mines3

 Average base case LOM annual FCF of $158 million ($296 million per year in the first 5 years)

 Improved single-phase build over 24 months and direct ramp -up to 4,900 tpd with total initial

capital cost of $881 million and sustaining capital of $525 million over the LOM

 Streamlined processing facilities into a single location and improved flowsheet design with

incorporation of a gravity circuit and production of higher-grade concentrate product

 Strong support for local employment with up to 613 direct jobs created during peak construction

and 525 permanent jobs during operations

 Significant opportunities to potentially enhance Project economics and extend mine life through

conversion of Mineral Resources adjacent to Mineral Reserves through infill drilling

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and positive outlook , we believe this project is well -positioned to deliver substantial value to all

stakeholders. We look forward to sharing further updates in the coming months."

A formal positive final investment decision , along with securing of a project financing package in the

coming months would enable certain construction activities to commence in the second half of 2025 ,

with project completion targeted for the end of 2027.

Osisko Development will host a conference call and webinar presentation by management on the 2025

FS results on Monday, April 28, 2025 at 11:00 a.m. ET, followed by a question & answer session .

Details for dial-in, webcast access, and replay archive are available at the end of this news release.

OPTIMIZED FEASIBILITY STUDY OVERVIEW

The Cariboo Gold Project is envisioned as a traditional underground operation, employing mechanized

long-hole open stoping to extract ore from gold -bearing vein corridors —an intricate network of

mineralized quartz veins predominantly hosted within unmineralized sandstone. An improved flowsheet

from the 2023 FS (as defined herein 5), supported by additional metallurgical testwork, envisions o re

beneficiation to be exclusively completed at the Mine Site Complex. This would produce saleable gold

doré from a gravity concentrate and ~66 tpd of high-grade flotation concentrate averaging ~133 g/t

Au. The flotation concentrate would be transported by truck to the Port of Vancouver for transport and

sale to a smelting partner. 2025 FS key summary results and assumptions are outlined in Table 1:

Table 1: Cariboo Gold 2025 FS – Key Results and Assumptions (after-tax)

Metric units Base Case Spot Case

Gold price US$/oz $2,400 $3,300

Exchange rate USDCAD 1.35 1.40

Net Present Value at 5% discount $ mm 943 2,066

Internal Rate of Return (IRR) % 22.1% 38.0%

Payback, from commercial production years 2.8 1.6

Average annual free cash flow1 $ mm 158 314

Average AISC, LOM1 US$/oz 1,157 1,167

1. All-in sustaining costs per ounce and free cash flow are non-IFRS measures or ratios. Refer to "Non-IFRS Financial Measures" at

the end of this news release for more information.

2. Spot case is based on the LBMA gold price as of the close of business on April 23, 2025, rounded to nearest $100/oz and the

USDCAD exchange rate is based on the Bank of Canada daily exchange rate, rounded to nearest five cents.

Key Improvements and Optimizations vs. 2023 FS

The 2025 FS incorporates several important improvements and de -risking initiatives over the 2023

Feasibility Study that better position the Project from an execution , financing, and operational

perspective. Notable changes include:

• Accelerated Development S equence: Single-phase construction and ramp up directly to

nameplate capacity of 4,900 tpd, which increases the LOM average gold production profile by

16% to 190,000 oz per year, and 202,000 oz per year in the first five years.

• Streamlined Processing: A single milling facility at the mine site removes the need (as had

been previously contemplated) to transport flotation concentrate 116 km to the QR Mill . This

reduces capital and operating costs by consolidating operations into one location.

• Improved Flowsheet Design: Updated metallurgical studies and testing has resulted in the

addition of a gravity circuit which, combined with a rougher and cleaner flotation circuit, resulted

in overall project gold recovery of 92.6% and the production of ~66 tpd higher-grade

concentrate product (reduction from 590 tpd in 2023 FS Phase II) averaging ~133 g/t Au (vs.

28 g/t Au in 2023 FS Phase II). Approximately 46% of gold is expected to be recovered by

gravity.

• Underground Mine Design: Increased average stope size by ~60% compared to the 2023 FS,

significantly reducing the total number of stopes required to achieve average daily throughput.

Optimization of the geotechnical design of the mined stopes, supported by recent trial mining,

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allows for more operational flexibility of underground operations.

• Mineral Reserves: Probable Mineral Reserves remained largely unchanged, increasing slightly

to 2.071 million ounces Au (17.8 Mt grading 3.62 g/t Au).

• Permitted: Project design and sequencing contemplated in the 2025 FS is aligned with the BC

Mines Act and Environmental Management Act permits obtained in Q4 2024.

Table 2: Cariboo Gold 2025 FS vs. 2023 FS – Project Operating and Financial Metrics

Assumptions units 2023 FS 2025 FS

Gold price US$/oz 1,700 2,400

Exchange rate USDCAD 1.30 1.35

Discount rate % 5.0% 5.0%

Production

Mine life yrs 12.0 10.0

Total ore mined kt 16,703 17,815

Peak annual throughput tpd 4,900 4,900

Average gold head grade g/t Au 3.78 3.62

Total contained gold koz 2,031 2,071

Avg. gold recovery % 92.0% 92.6%

Total recovered gold, payable koz 1,869 1,894

Avg. gold production, LOM koz/yr 164 190

Avg. gold production, first 5 yrs koz/yr 96 202

Operating Unit Costs

Underground mining $/t mined 53.6 62.3

Processing $/t mined 31.2 23.2

Water and waste management $/t mined 7.2 5.0

Electrical transmission line $/t mined — 4.9

General and administrative $/t mined 10.7 15.4

Total unit operating costs $/t mined 102.6 110.7

Total operating costs $ mm 1,714 1,921

Royalty payments $ mm 206 292

Offsite charges $ mm 5 143

Operating Costs

Total cash costs2 US$/oz $792 $947

AISC2 US$/oz $968 $1,157

Capital Expenditures

Initial costs $ mm 137.3 881

Expansion costs $ mm 451.1 —

Sustaining costs $ mm 466.6 426

Closure costs, net3 $ mm (38.9) 99

Total capex $ mm 1,016 1,406

Economics (after-tax)

Total free cash flow, LOM2 $ mm 901 1,577

Net Present Value (NPV5%) $ mm 502 943

Internal Rate of Return (IRR) % 20.7% 22.1%

Payback, from commercial production yrs 5.9 2.8

Average free cash flow, first 5 yrs2 $ mm (63) 296

Average free cash flow, LOM2 $ mm 75 158

1. Total may not add up due to rounding.

2. Cash costs, all-in sustaining costs per ounce and free cash flow are non -IFRS measures or ratios. Refer to "Non-IFRS Financial

Measures" at the end of this news release for more information. Total cash costs are presented on a per ounce payable basis

inclusive of total operating costs mining costs, processing costs, site G&A costs, royalties, smelting, refining, and transports

costs. AISC are presented on a per ounce payable basis and include cash costs plus sustaining and closure costs.

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3. Closure costs are shown net of salvage value.

4. Pre-final investment decision capital costs total $38.6 million.

Figure 1: Payable Gold Production and AISC1 Profile by Year

1. All-in sustaining costs per ounce is a non-IFRS ratio. Refer to "Non-IFRS Financial Measures" at the end of this news release for

more information.

Figure 2: Free Cash Flow1 by Year

1. Free cash flow is a non -IFRS measure. Refer to "Non -IFRS Financial Measures" at the end of this news release for more

information.

ECONOMIC SENSITIVITY ANALYSIS

The Company used a base case gold price assumption of US$2,400/oz and a USDCAD exchange rate of

1.35 in its analysis and incorporated only Probable Mineral Reserves (as defined herein). Based on these

50

199

223

207 207

176 181 183 177 171

112

–

$250

$500

$750

$1,000

$1,250

$1,500

–

50

100

150

200

250

-1 1 2 3 4 5 6 7 8 9 10

AISC (US$/oz)

Gold Production (koz)

Year

Gold Payable Production (lhs) AISC (rhs)

($500)

($400)

($300)

($200)

($100)

–

$100

$200

$300

$400

$500

$600

-3 -2 -1 1 2 3 4 5 6 7 8 9 10

C$ million

Year

Base Case Spot Case

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assumptions, the Project generates an after-tax NPV5% of $943 million, an after-tax unlevered IRR of

22.1% and a payback of 2.8 years (from commercial production). The 2025 FS economics are most

sensitive to fluctuations in the following inputs (in order of magnitude of impact ): gold price, foreign

exchange, operating costs, and capital costs.

Table 3: Cariboo Gold 2025 FS – Economic Sensitivity Analysis, after-tax (base case in bold)

Gold Price NPV5% IRR Payback Avg. FCF

(US$/oz) ($ mm) (%) (yrs) ($ mm per yr)

$1,800 251 9.9% 5.3 65

$2,100 610 16.5% 3.5 113

$2,400 943 22.1% 2.8 158

$2,700 1,275 27.2% 2.2 204

$3,000 1,604 31.9% 1.9 250

$3,300 1,934 36.3% 1.7 296

$3,600 2,263 40.5% 1.5 342

$3,900 2,592 44.5% 1.3 388

$4,200 2,921 48.3% 1.2 433

$4,500 3,249 51.9% 1.1 479

Table 4: Cariboo Gold 2025 FS – NPV5% Sensitivity Analysis, after-tax, $ mm (base case in bold)

Gold Price USDCAD Exchange Rate

(US$/oz) 1.25 1.30 1.35 1.40 1.45

$1,800 59 166 251 335 419

$2,100 439 525 610 695 780

$2,400 750 847 943 1,040 1,136

$2,700 1,058 1,167 1,275 1,382 1,490

$3,000 1,364 1,484 1,604 1,724 1,845

$3,300 1,669 1,801 1,934 2,066 2,198

$3,600 1,974 2,118 2,263 2,408 2,552

$3,900 2,279 2,436 2,592 2,749 2,905

$4,200 2,584 2,753 2,921 3,089 3,258

$4,500 2,888 3,069 3,249 3,430 3,610

Table 5: Cariboo Gold 2025 FS – IRR Sensitivity Analysis, after-tax, % (base case in bold)

Gold Price USDCAD Exchange Rate

(US$/oz) 1.25 1.30 1.35 1.40 1.45

$1,800 6.2% 8.3% 9.9% 11.5% 13.0%

$2,100 13.4% 15.0% 16.5% 18.0% 19.4%

$2,400 18.9% 20.6% 22.1% 23.7% 25.2%

$2,700 23.9% 25.6% 27.2% 28.8% 30.3%

$3,000 28.5% 30.2% 31.9% 33.5% 35.2%

$3,300 32.8% 34.6% 36.3% 38.0% 39.7%

$3,600 36.8% 38.7% 40.5% 42.3% 44.1%

$3,900 40.7% 42.7% 44.5% 46.3% 48.1%

$4,200 44.4% 46.4% 48.3% 50.1% 52.0%

$4,500 47.9% 49.9% 51.9% 53.8% 55.7%

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MINERAL RESOURCES AND MINERAL RESERVES

Vein corridors that make up the Cariboo Gold Project Mineral Resources and Mineral Reserves comprise

a high-density network of mineralized quartz veins hosted within unmineralized sandstone. Individual

mineralized veins within these corridors have widths varying from centimeters to several meters and

strike lengths from a few meters to over 50 m. These corridors have been defined from surface to a

vertical depth of 650 m, averaging 300m, and remain open for expansion at depth and along strike.

Gold grades are intimately associated with quartz vein-hosted pyrite as well as pyritic, intensely silicified

wall rock haloes in close proximity to the veins.

Figure 3: Vein Corridor in Lowhee Bulk Sample Area

Mineral Resources Estimate

The Mineral Resources estimate included in the 2025 FS have an effective date of April 22, 2025, and

are reported exclusive of Mineral Reserves. Mineral Resources that are not Mineral Reserves do not have

demonstrated economic viability. The Mineral Resources estimate have been updated to account for

depletion in the Lowhee Zone, due to ongoing development and bulk sample activities, as well as for

changes in costs and cut -off grade assumptions. This resulted in a n aggregate increase of 3% of the

total contained gold ounces in the Indicated Resources category a nd an increase of 8% of the total

contained gold ounces in the Inferred Resources category as compared to the 2023 FS.

There is no certainty that Mineral Resources will be converted into Mineral Reserves. Mineral Resources

include Inferred Mineral Resources which have had insufficient work to classify them as Indicated Mineral

Resources. It is uncertain but reasonably expected that Inferred Mineral Resources could be upgraded

to Indicated Mineral Resources with continued exploration.

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Table 6: Cariboo Gold 2025 FS – Mineral Resources Statement (April 22, 2025)

Category / Zone Tonnage Gold Grade Contained Gold

(000's tonnes) (g/t) (000's oz)

Measured

Bonanza Ledge 47 5.06 8

Indicated

Bonanza Ledge 32 4.02 4

BC Vein 1,057 3.00 102

KL 527 2.80 47

Lowhee 1,333 2.76 118

Mosquito 1,553 2.96 148

Shaft 6,121 2.92 575

Valley 2,718 2.70 236

Cow 3,991 2.91 374

Total Indicated Resources 17,332 2.88 1,604

Total Measured & Indicated 17,380 2.88 1,612

Inferred

BC Vein 596 3.17 61

KL 2,514 2.53 205

Lowhee 486 3.01 47

Mosquito 1,883 3.08 186

Shaft 7,457 3.44 826

Valley 2,470 3.01 239

Cow 3,368 2.78 301

Total Inferred Resources 18,774 3.09 1,864

1. The independent and qualified persons for the Mineral Resources estimates, as defined by NI 43-101, are Carl Pelletier, P.Geo.,

and Tessa Scott, P.Geo. (Norda Stelo). The effective date of the 202 5 Feasibility Study Mineral Resource Estimate is April 22,

2025.

2. These Mineral Resources, exclusive of the reserves, are not Mineral Reserves and do not have demonstrated economic viability.

3. The Mineral Resources estimate follows the 2014 CIM Definition Standards on Mineral Resources and Reserves and the 2019

CIM Estimation of Mineral Resources and Mineral Reserves Best Practice Guidelines.

4. A total of 481 vein zones were modelled for the Cow Mountain (Cow and Valley), Island Mountain (Shaft and Mosquito),

Barkerville Mountain (BC Vein, KL, and Lowhee) deposits and one gold zone for Bonanza Ledge. A minimum true thickness of

2.0 m was applied, using the gold grade of the adjacent material when assayed or a value of zero when not assayed.

5. The estimate is reported for a potential underground scenario at a cut -off grade of 1.8 g/t Au, except for Bonanza Ledge at a

cut-off grade of 3.5 g/t Au. The cut -off grade for the Cow, Valley, Shaft, Mosquito, BC Vein, KL, and Lowhee deposits was

calculated using a gold price of US$2,400/oz; a USDCAD exchange rate of 1.35; an underground mining cost of $66.3/t; a

processing and transport cost of $30.80/t; a G&A plus Environmental cost of $22.40/t; and a sustaining CAPEX cost of $45.6/t.

No changes have been applied for the Bonanza Ledge. The cut-off grade for the Bonanza Ledge deposit was calculated using a

gold price of US$1,700/oz; a USDCAD exchange rate of 1. 27; an underground mining cost of $79.13/t; a processing and

transport cost of $65.00/t; and a G&A plus Environmental cost of $51.65/t. The cut -off grades may be re-evaluated in light of

future prevailing market conditions (metal prices, exchange rate, mining cost, etc.).

6. Density values for Cow, Shaft, Lowhee, and BC Vein were estimated using the ID2 interpolation method, with a value applied

for the non-estimated blocks of 2.80 g/cm3 for Cow, 2.78 g/cm3 for Shaft, 2.74 g/cm3 for Lowhee, and 2.69 g/cm3 for BC Vein.

Median densities were applied for Valley (2.81 g/cm 3), Mosquito (2.79 g/cm 3), and KL (2.81 g/cm 3). A density of 3.20 g/cm 3

was applied for Bonanza Ledge.

7. A four-step capping procedure was applied to composited data for Cow (3.0 m), Valley (1.5 m), Shaft (2.0 m), Mosquito (2.5

m), BC Vein (2.0 m), KL (1.75 m), and Lowhee (1.5 m). Restricted search ellipsoids ranged from 7 to 50 g/t Au at four different

distances ranging from 25 m to 250 m for each deposit. High grades at Bonanza Ledge were capped at 70 g/t Au on 2.0 m

composited data.

8. The gold Mineral Resources for the Cow, Valley, Shaft, Mosquito, BC Vein, KL, and Lowhee vein zones were estimated using

Datamine StudioTM RM 1.9 software using hard boundaries on composited assays. The dilution halo gold mineralization w as

estimated using Datamine StudioTM RM Pro 1.11. The OK method was used to interpolate a sub -blocked model (parent block

size = 5 m x 5 m x 5 m). Mineral Resources for Bonanza Ledge were estimated using GEOVIA GEMSTM 6.7 software using hard

boundaries on composited assays. The OK method was used to interpolate a block model (block size = 2 m x 2 m x 5 m).

9. Results are presented in situ. Ounce (troy) = metric tons x grade / 31.10348. Calculations used metric units (metres, tonnes,

g/t). The number of tonnes were rounded to the nearest thousand. Any discrepancies in the totals are due to rounding effects.

Rounding followed the recommendations as per NI 43-101.

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10. The qualified persons responsible for this section of the technical report are not aware of any environmental, permitting, le gal,

title, taxation, socio-economic, marketing, political, or other relevant factors that could materially affect the Mineral Reso urce

estimate other than those disclosed in this news release and in the Technical Report.

Mineral Reserves Estimate

The 2025 FS outlines Probable Mineral Reserves estimate (with an effective date of April 10, 2025)

which is increased slightly from the 2023 FS due to the adjustment of the cut-off-grade dictated by an

accelerated ramp-up schedule to 4,900 tpd throughput relative to the 2023 FS.

Table 7: Cariboo Gold 2025 FS – Mineral Reserves Statement (April 10, 2025)

Category / Zone Tonnage Gold Grade Contained Gold

(tonnes) (g/t) (oz)

Proven — — —

Probable

Cow 3,999,971 3.35 430,548

Valley 3,238,636 3.59 374,058

Shaft 8,548,295 3.72 1,021,599

Mosquito 1,105,370 3.94 140,102

Lowhee 923,162 3.52 104,491

Total Proven & Probable 17,815,435 3.62 2,070,798

1. Totals may not add up due to rounding.

2. The Mineral Reserve estimate follows the 2014 CIM Definition Standards on Mineral Resources and Reserves and the 2019 CIM

Estimation of Mineral Resources and Mineral Reserves Best Practice Guidelines.

3. Mineral Reserves used the following assumptions: US$1,915/oz gold price, USD:CAD exchange rate of 1.32, and variable cut -

off value from 1.70 g/t to 2.0 g/t Au

4. Mineral Reserves include both internal and external dilution along with mining recovery. The external dilution is estimated to be

10.1%. The average mining recovery factor was set at 91.3% to account for ore left in each block in the margins of the deposit.

PRODUCTION PROFILE

The 2025 FS outlines an average annual gold production profile of 190,000 ounces over a 10-year mine

life. Total payable gold recovered is 1.894 million ounces with an average head grade of 3.62 g/t Au,

total metallurgical recovery of 92.6% and a 97.75% concentrate payability factor.

Figure 4: Recovered Gold and Grade Profile by Year

51

201

226

209 210

178 184 186 179 173

113

3.90 3.81

4.16

3.91 3.96

3.36

3.52 3.50 3.40 3.29

3.00

–

1.00

2.00

3.00

4.00

5.00

–

25

50

75

100

125

150

175

200

225

-1 1 2 3 4 5 6 7 8 9 10

Head Grade (g/t Au)

Gold Production (koz)

Year

Gold Recovered (lhs) Head Grade (rhs)