Osisko Development Provides Infill Drilling Update ON Its 13,000-m Program at Cariboo GOLD Project; Intercepts Include 8.56 G/T Au over 8.5m from 60.5m Depth, and 130.32 G/T Au over 0.5m from 83.5m Depth

ODV NYSE TSXV News Release

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OSISKO DEVELOPMENT PROVIDES INFILL DRILLING UPDATE ON ITS 13,000-m

PROGRAM AT CARIBOO GOLD PROJECT; INTERCEPTS INCLUDE 8.56 g/t AU OVER

8.5m FROM 60.5m DEPTH, AND 130.32 g/t AU OVER 0.5m FROM 83.5m DEPTH

Montreal, Québec, December 15, 2025 – Osisko Development Corp. (NYSE: ODV, TSXV: ODV)

("Osisko Development" or the "Company") is pleased to announce new infill drilling results from its

ongoing 13,000-meter program on 10 meter drill spacing that commenced in August 2025 in the Lowhee

Zone of the Company's permitted, 100%-owned Cariboo Gold Project (" Cariboo" or the " Project"),

located in central British Columbia ("B.C."), Canada. The six drill hole fans reported herein comprise an

additional ~3,704 meters ("m") of underground infill drilling, bringing the total tally of drilling with full

results to 5,983 m, or ~44% of the planned total.

In total, a n aggregate of approximately 10,000 meters, representing ~72% of the total planned drill

meters, has now been completed. Full assay results and associated quality assurance and quality control

reviews are pending for the remaining holes not reported herein.

Chris Lodder, President, stated, "We continue to see good alignment of ongoing infill drilling results

with our modelled vein corridors and planned stope shapes, along with occurrences of upside mineralized

zones outside current reserve outlines. This tighter drill spacing sharpens our understanding of vein

corridor spatial geometries and local variability , while reinforcing the importance of infill drilling in

underexplored zones. As this work concludes in the first quarter of 2026, the resulting information is

expected to help refine production designs and sequencing."

DRILL ASSAY HIGHLIGHTS

This news release includes assays from thirty-eight (38) underground infill HQ diamond drill ("DD")

holes (63.5-millimeter diameter) totaling ~3,704 m with depths ranging from 81 to 114 m completed

between September 2025 and early November 2025 (see Table 1). Assays for five (5) complete drill

fans were received by November 27, 2025 (Figure 2). Select photon assay highlights include:

• 8.56 grams per tonne ("g/t") gold ("Au") over 8.5 m from 60.5 m depth in hole BMU-

25-090, including:

o 64.26 g/t over 0.5 m from 63.5 m depth, and

o 43.80 g/t over 0.5 m from 62.5 m depth, and

o 12.86 g/t over 0.5 m from 62 m depth, and

o 10.71 g/t over 0.5 m from 61 m depth, and

o 3.35 g/t over 0.5 m from 66.5 m depth, and

HIGHLIGHTS

 5,983 m of infill drilling completed (3,704 m of new results) on 10-m drill spacing in 62 drill holes

from August 2025, representing ~44% of the total planned 13,000-m infill program

 Highlights include: 8.56 g/t Au over 8.5m from 60.5m depth, 16.95 g/t Au over 4.0m from 80m depth

(including 130.32 g/t Au over 0.5m from 83.5m depth), and 9.32 g/t Au over 6.0m from 58m depth

 Results to date continue to be broadly consistent with modelled reserve stopes, indicate potential

for upside mineralization in new areas , and provide key spatial controls that will inform an

updated local block model and refine future production stope designs

 An aggregate total of 10,000 m (~72%) of total planned drill meters ha ve been completed to

date, with full assays and QA/QC pending for the remaining holes not reported

 Infill program on track to wrap up in Q1 2026

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o 2.69 g/t over 0.5 m from 63 m depth, and

o 2.67 g/t over 0.5 m from 64 m depth

• 16.95 g/t over 4.0 m from 80 m depth in BMU-25-104, including:

o 130.32 g/t over 0.5 m from 83.5 m depth, and

o 4.37 g/t over 0.5 m from 80 m depth

• 9.32 g/t over 6.0 m from 58 m depth in BMU-25-092, including:

o 51.09 g/t over 0.5 m from 63 m depth, and

o 26.51 g/t over 0.5 m from 62 m depth, and

o 23.52 g/t over 0.5 m from 59.7 m depth, and

o 3.10 g/t over 0.7 m from 60.2 m depth, and

o 3.21 g/t over 0.5 m from 60.9 m depth

• 13.22 g/t over 2.65 m from 87.85 m depth in BMU-25-108, including:

o 65.63 g/t over 0.5 m from 90 m depth, and

o 3.97 g/t over 0.5 m from 87.85 m depth

• 26.14 g/t over 2.0 m from 2.5 m depth in BMU-25-110, including:

o 103.94 g/t over 0.5 m from 3 m depth

• 11.12 g/t over 4.5 m from 70.5 m depth in BMU-25-084, including:

o 88.87 g/t over 0.5 m from 71.5 m depth, and

o 7.08 g/t over 0.5 m from 72 m depth, and

o 4.76 g/t over 0.5 m from 76.5 m depth

• 11.23 g/t over 3.9 m from 59.1 m depth in BMU-25-087, including:

o 43.0 g/t over 1.0 m from 60.8 m depth

• 7.32 g/t over 4.0 m from 15 m depth in BMU-25-101, including:

o 57.72 g/t over 0.5 m from 16.5 m depth, and

o 1.87 g/t over 0.5 m from 17.5 m depth

• 31.32 g/t over 1.0 m from 42.9 m depth in BMU-25-104, including:

o 62.15 g/t over 0.5 m from 42.9 m depth

• 6.48 g/t over 4.55 m from 50.45 m depth in BMU-25-108, including:

o 21.04 g/t over 0.5 m from 51.45 m depth, and

o 12.18 g/t over 0.5 m from 51.95 m depth, and

o 8.71 g/t over 0.55 m from 53.45 m depth, and

o 7.0 g/t over 0.5 m from 54 m depth, and

o 4.41 g/t over 0.5 m from 50.95 m depth, and

o 2.88 g/t over 0.5 m from 52.95 m depth

Complete assay highlights, including true width estimates, are presented in Table 1 and drill hole

locations and orientations are listed in Table 2. Intervals not recovered by drilling were assigned zero

grade. Top cuts have not been applied to high grade assays.

DISCUSSION OF RESULTS

• Based on the results observed to date, there is a positive correlation between the above cut-off

assay composites and the modelled reserve stopes. Individual intercepts are not expected to

align precisely with the modelled areas, and a degree of variability within the vein corridors is

both anticipated and acceptable as tighter spaced infill drilling informs a refinement of the local

reserve model. This work is one of the key objectives of the ongoing program.

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• Occurrences of above cut-off assay are also being observed in areas not previously included in

the reserve model, suggesting potential for ups ide mineralization. These intervals will be

incorporated in the planned remodelling and mineral resource calculation process to determine

their implications on the updated local block model and any potential adjustments to planned

reserve stopes. In certain areas, this may support the addition of new planned reserve stopes,

subject to the final estimation process outcome.

• A spatial offset of certain intercepts when compared to the modelled vein corridors and mineral

reserve stope shapes is evident in the cross sections. This reflects, in part, the lower spatial

accuracy of the surface-based drill hole data compared to underground drilling , which carries

higher survey precision, as well as the more oblique angles of intercepts that the surface-based

drill holes have with the vein planes. These factors will be accounted for in the vein corridor

remodelling process to be undertaken upon conclusion of this program , and will serve as an

important operational template for future infill drilling used in production stope design.

Figure 1: Cariboo Gold Project deposit map with Location of Lowhee Zone and Cow Portal underground access.

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Figure 2: Location and overview of the ongoing 13,000-meter infill drilling campaign.

Figure 3: Lowhee Zone infill select underground drilling highlights (plan view).

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Figure 4: Lowhee Zone infill select underground drill assay highlights (this release) with previously released

surface and underground diamond drilling results in cross section by fan.

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