Osisko Development Intercepts 57.29 G/T GOLD over 3.05 Meters, 625.95 G/T GOLD over 0.5 Meters and 15.26 G/T GOLD over 4.4 Meters IN Lowhee Infill Drilling at Cariboo GOLD Project

ODV NYSE TSXV News Release

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OSISKO DEVELOPMENT INTERCEPTS 57.29 g/t GOLD OVER 3.05 METERS, 625.95

g/t GOLD OVER 0.5 METERS AND 15.26 g/t GOLD OVER 4.4 METERS IN LOWHEE

INFILL DRILLING AT CARIBOO GOLD PROJECT

Montreal, Québec, September 8, 2025 – Osisko Development Corp. (NYSE: ODV, TSXV: ODV)

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

exploration diamond drill , and development sampling campaigns in the Lowhee Zone within the

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

central British Columbia (" B.C."), Canada. The program to date has consisted of approximately 6,471

meters ("m") of underground infill drilling and approximately 398 m of chip and rock saw channel

sampling.

Chris Lodder, President, stated, "The recent underground infill drilling at Lowhee Zone was conducted

to analyze how grade holds up under tighter drilling and to investigate optimal drill hole spacing for

future mining. We're highly encouraged that u nderground exploration drilling in the Lowhee Zone and

across the deposit completed to date continues to indicate that veins remain open at depth, highlighting

the strong potential for future resource expansion through additional exploration programs leveraging

existing underground infrastructure. A comprehensive 13,000 -meter infill drill program is already

underway, designed to deliver important data for local resource modeling, mine planning, and

optimization of production stope design . We anticipate providing regular progress updates on this

program in the coming months."

DRILL ASSAY HIGHLIGHTS

This news release includes assays from sixty-eight (68) underground infill and exploration diamond drill

("DD") holes totaling approximately 6,471 m completed from November 2024 through early August

2025 (see Table 1) and for which assays were received by a cut off date of August 16, 2025. Select fire

and photon assay highlights include:

• 625.95 grams per tonne ("g/t") gold ("Au") over 0.5 m in hole BMU-25-040

• 57.29 g/t Au over 3.05 m in hole BMU-25-032, including:

o 238.20 g/t Au over 0.5 m

o 52.57 g/t Au over 0.6 m

o 23.77 g/t Au over 1.0 m

• 15.26 g/t Au over 4.4 m in hole BMU-25-004, including:

o 83.40 g/t au over 0.5 m

o 46.80 g/t Au over 0.5 m

• 26.23 g/t Au over 2.4 m in hole BMU-25-025 including:

o 55.96 g/t Au over 0.8 m

o 29.45 g/t Au over 0.55 m

o 3.39 g/t Au over 0.5 m

• 55.74 g/t Au over 1.0 m in hole BMU-25-034, including:

o 57.15 g/t Au over 0.5 m

o 54.34 g/t Au over 0.5 m

• 8.37 g/t Au over 5.9 m in hole BMU-25-014, including:

o 50.24 g/t Au over 0.5 m

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o 15.97 g/t Au over 0.5 m

o 10.02 g/t Au over 0.55 m

o 6.88 g/t Au over 0.5 m

o 6.41 g/t Au over 0.5 m

o 5.74 g/t Au over 0.5 m

• 7.06 g/t Au over 6.4 m in hole BMU-25-026, including:

o 75.61 g/t Au over 0.5 m

o 8.53 g/t Au over 0.5 m

• 12.44 g/t Au over 3.45 m in hole BMU-25-017, including:

o 79.17 g/t Au over 0.5 m

o 4.93 g/t Au over 0.5 m

• 7.23 g/t Au over 5.75 m in hole BMU-25-019, including:

o 61.18 g/t Au over 0.5 m

o 13.32 g/t Au over 0.5 m

o 3.31 g/t Au over 0.5 m

• 28.55 g/t Au over 1.45 m in hole BMU-25-045, including:

o 80.20 g/t Au over 0.5 m

True widths are estimated to be between 50% and 90% of reported core length intervals. Intervals not

recovered by drilling were assigned zero grade. Top cuts have not been applied to high grade assays.

Complete assay highlights are presented in Table 1 and drill hole locations and orientations are listed in

Table 3.

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

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Figure 2: Lowhee Zone select underground drilling highlights.

INFILL AND EXPLORATION PROGRAM SUMMARY

• 14 NQ (47.6-millimeter diameter) diamond drill holes (BMU-25-008 to 021) with depth ranging

from 42 to 48 m focused on infilling a 25 m long section of planned ore drive.

• 4 NQ diamond drill holes (BMU -25-001, 003, 004, 007) with an average depth of 290 m were

drilled to explore zones adjacent to the existing Lowhee Zone mineral resources.

• 22 NQ diamond drill holes (BMU -25-032 to 053) with an average depth of 90 m conducting

systematic grid infill on a set of mineral reserve blocks, mineral inferred resources, and non-

DSO resources.

• 8 NQ diamond drill holes (BMU -24-001 to 008) with an average depth of 118 m were drilled

infilling areas within existing mineral reserves and resources.

NEXT STEPS

• A 13,000-meter systematic grid infill program is underway within the Lowhee Zone targeting

mineral reserve blocks, inferred mineral resources, and non-DSO resources in an approximate

100 m swath transecting approximately the full width of the Lowhee Zone (Figure 3). Drilling at

10 m centers from new development at the 1 ,260 elevation level, the program is expected to

provide a comprehensive data set that will inform resource modeling, mine planning, production

stope design procedures and parameters , and the appropriate grid spacing for future infill

drilling. Additional systematic grid infill drill programs are planned in step with future

underground development progress into other areas of the deposit.

• Structural modelling suggests that the sandstone rock package, the primary host rock for the

Cariboo deposit, remains open well beyond depths tested to date. Positive results from deeper

drilling across the deposit highlight strong potential for extensions at depth, below the currently

defined mineral reserves and resources (Figure 4).

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Figure 3: Location of the ongoing 13,000-meter infill drilling campaign.

Figure 4: Northeast facing long section of the Cow Mountain and Lowhee Zone deposit areas with select three-meter

composite highlights illustrating exploration potential at depth and along strike to the southeast.

UNDERGROUND CHIP/CHANNEL SAMPLING HIGHLIGHTS

This news release contains assays from eighty-nine (89) underground face and wall rock chip and rock

saw channels ("CH") totaling approximately 398 m completed in ore and access development from

October 2024 through July 2025 (see Table 2) and for which assays were received by a cut off date of

July 31, 2025. Select fire and photon assay highlights include:

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• 22.01 g/t Au over 3.8 m in chip channel UGCH00189, including:

o 104.37 g/t Au over 0.8 m

• 12.23 g/t Au over 4.6 m in chip channel UGCH00183, including:

o 69.95 g/t Au over 0.8 m

• 6.25 g/t Au over 4.2 m in rock saw channel UGCH00236, including:

o 12.64 g/t Au over 1.0 m

o 13.62 g/t Au over 0.9 m

• 5.81 g/t Au over 4.2 m in face chip channel UGCH00193, including:

o 20.66 g/t Au over 1.1 m

• 3.32 g/t Au over 6.5 m in face chip channel UGCH00213, including:

o 13.52 g/t Au over 1.0 m

o 7.77 g/t Au over 1.0 m

• 4.82 g/t Au over 4.2 m in rock saw channel UGCH00246, including:

o 25.22 g/t Au over 0.8 m

• 4.22 g/t Au over 4.2 m in rock saw channel UGCH00245, including:

o 7.51 g/t Au over 1.2 m

o 4.86 g/t Au over 1.5 m

• 3.61 g/t Au over 4.4 m in chip channel UGCH00176, including:

o 22.20 g/t Au over 0.7 m

• 3.02 g/t Au over 5.2 m in chip channel UGCH00210, including:

o 9.87 g/t Au over 1.0 m

o 3.13 g/t Au over 1.2 m

• 4.14 g/t Au over 3.7 m in rock saw channel UGCH00243, including:

o 7.65 g/t Au over 1.0 m

o 6.07 g/t Au over 1.2 m

True widths are estimated to be between 85% and 100% of reported sample intervals for all face

sampling in ore drives and wall sampling in crosscuts. Wall samples in ore drives (along the vein) are

excluded from the above list and indicated as select (Table 4). Top cuts have not been applied to high

grade assays. Complete assay highlights are presented in Table 2 and sample locations are listed in

Table 4.

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Figure 5: Lowhee Zone face and wall rock chip and rock saw channel samples with select assay highlights.

Table 1: Length weighted assay composites and individual samples>=3.0 g/t for Lowhee Zone underground diamond

drillholes completed November 2024 – August 16, 2025.

Drillhole ID From (m) To (m) Length (m) Au (g/t)

BMU-24-001 77.8 78.3 0.5 6.28

83.4 85.25 1.85 9.64

Including 84.25 84.75 0.5 33.54

92.5 93 0.5 3.25

102 105 3 7.89

Including 103.9 104.5 0.6 25.93

and 104.5 105 0.5 15.25

BMU-24-002 92.2 92.9 0.7 5.09

98 98.5 0.5 22.89

BMU-24-003 83 83.5 0.5 36.82

BMU-24-004 92.9 98 5.1 3.60

Including 92.9 93.4 0.5 6.10

and 95.6 96.1 0.5 24.06

BMU-24-005 117 117.5 1 4.28

123.75 124.25 0.5 17.85

BMU-24-006 80.4 82 1.6 3.72

Including 81 82 1 5.39

BMU-24-007 15.6 17.5 1.9 4.04

Including 17 17.5 0.5 12.51

34.9 35.5 0.6 3.81

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Drillhole ID From (m) To (m) Length (m) Au (g/t)

39.2 39.7 0.5 17.30

47.8 48.3 0.5 6.92

87 87.55 0.55 3.05

94.1 94.6 0.5 19.34

100.5 104.05 3.55 3.40

Including 100.5 101.4 0.9 7.53

and 102 102.5 0.5 3.16

and 103.4 104.05 0.65 3.94

106.9 107.7 0.8 3.70

111.45 113.2 1.75 12.18

Including 111.45 111.95 0.5 23.82

and 111.95 112.65 0.7 12.03

BMU-24-008 20.75 22 1.25 3.70

Including 20.75 21.25 0.5 8.48

53.5 55.25 1.75 3.76

Including 53.5 54.05 0.55 10.15

100.55 101.6 1.05 25.19

104.6 105.1 0.5 23.47

109.4 112.5 3.1 3.85

Including 109.9 110.5 0.6 6.62

and 112 112.5 0.5 12.14

118.8 119.4 0.6 5.59

BMU-25-001 130.5 133.65 3.15 7.29

Including 132.5 133.65 1.15 18.45

BMU-25-002 No significant assays

BMU-25-003 273 273.5 0.5 7.64

BMU-25-004 15 15.5 0.5 3.22

24.75 25.3 0.55 3.87

46.2 47.25 1.05 4.09

Including 46.75 47.25 0.5 7.89

64.75 67.45 2.7 5.51

Including 64.75 65.25 0.5 19.35

and 66.85 67.45 0.6 8.53

84.45 85.35 0.9 17.90

95.5 96.5 1 10.94

Including 96 96.5 0.5 21.10

116.5 120.2 3.7 8.00

Including 118 118.6 0.6 6.82

and 119.6 120.2 0.6 40.80

125.3 126 0.7 18.05

139.1 141.8 2.7 3.51

Including 139.6 140.15 0.55 3.44

and 141.25 141.8 0.55 11.85

144 148.4 4.4 15.26

Including 144 144.5 0.5 83.40

and 147.9 148.4 0.5 46.80

152.9 153.6 0.7 7.71

158.45 159 0.55 7.81

183.25 183.85 0.6 3.18

222.35 224 1.65 5.41

Including 222.35 223 0.65 12.40

BMU-25-005 Geotechnical hole; no significant assays

BMU-25-006 Geotechnical hole; not sampled

BMU-25-007 0.75 1.25 0.5 7.05

30 33.55 3.55 7.98

Including 30 30.5 0.5 6.28

and 31.8 32.35 0.55 42.50

76.2 76.7 0.5 4.50

90.55 93.3 2.75 3.40

Including 90.55 91.05 0.5 3.75

and 92.75 93.3 0.55 13.50

99.35 102.7 3.35 6.32

Including 99.35 99.9 0.55 12.60

and 99.9 100.5 0.6 10.55

and 102.2 102.7 0.5 15.75

106.85 110.1 3.25 5.08

Including 109.5 110.1 0.6 25.40

118.25 122.85 4.6 7.07

Including 118.25 118.8 0.55 33.50

and 122 122.85 0.85 15.90

129.65 130.15 0.5 9.55

144.55 145.05 0.5 3.06

188 188.5 0.5 4.58

243.45 243.95 0.5 57.60

BMU-25-008 0.85 1.95 1.1 10.09

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Drillhole ID From (m) To (m) Length (m) Au (g/t)

Including 0.85 1.35 0.5 4.93

and 1.35 1.95 0.6 14.38

40.35 41.05 0.7 5.37

BMU-25-009 No significant assays

BMU-25-010a Hole abandoned

BMU-25-010 1.25 1.75 0.5 9.95

20.35 21 0.65 4.77

26.85 27.4 0.55 8.04

36 37.2 1.2 3.99

Including 36 36.5 0.5 3.94

and 36.5 37.2 0.7 4.02

BMU-25-011 0.7 1.75 1.05 4.03

Including 0.7 1.25 0.55 3.65

and 1.25 1.75 0.5 4.46

19.75 20.25 0.5 3.71

BMU-25-012 0.7 1.25 0.55 8.27

24.3 25.45 1.15 16.11

Including 24.85 25.45 0.6 28.18

28.65 29.2 0.55 4.77

30.35 30.9 0.55 3.58

BMU-25-013 0.7 2.35 1.65 3.24

Including 0.7 1.3 0.6 4.77

19.25 19.75 0.5 3.40

22.5 23 0.5 40.00

27.5 33.5 6 3.24

Including 29.9 30.4 0.5 4.42

and 30.4 30.9 0.5 21.73

and 32.06 32.6 0.54 7.76

BMU-25-014 8.65 9.15 0.5 7.16

30.7 36.6 5.9 8.37

Including 30.7 31.2 0.5 5.74

and 31.2 31.75 0.55 10.02

and 31.75 32.25 0.5 50.24

and 33.35 33.85 0.5 15.97

and 35.1 35.6 0.5 6.88

and 36.1 36.6 0.5 6.41

BMU-25-015 22.75 24 1.25 18.10

Including 22.75 23.25 0.5 40.65

and 23.25 24 0.75 3.06

BMU-25-016 33.95 39 5.05 4.28

Including 35.5 36.25 0.75 14.55

and 36.25 36.8 0.55 8.05

and 38.3 39 0.7 5.35

BMU-25-017 22.35 24.35 2 8.93

Including 23.35 23.85 0.5 12.74

and 23.85 24.35 0.5 21.68

30 33.45 3.45 12.44

Including 30 30.5 0.5 4.93

and 32.95 33.45 0.5 79.17

35.4 35.9 0.5 3.75

BMU-25-018 8.5 9 0.5 11.87

27.35 32.65 5.3 3.07

Including 28.2 28.8 0.6 11.62

and 29.7 30.3 0.6 9.01

and 32.15 32.65 0.5 5.72

40.75 41.25 0.5 5.41

BMU-25-019 25.15 25.65 0.5 12.77

29.15 31 1.85 8.85

Including 29.95 30.45 0.5 28.45

33.85 39.6 5.75 7.23

Including 36.25 36.75 0.5 13.32

and 37.75 38.25 0.5 3.31

and 38.25 38.75 0.5 61.18

BMU-25-020 28.6 29.15 0.55 3.13

29.75 30.4 0.65 3.85

37.8 39 1.2 3.75

Including 37.8 38.45 0.65 4.51

42.4 43.7 1.3 5.18

Including 42.9 43.7 0.8 7.87

BMU-25-021 28 31.9 3.9 6.06

Including 29.3 30.2 0.9 4.09

and 30.2 30.85 0.65 23.06

BMU-25-022 37 41.5 4.5 4.36

Including 40.5 41.5 1 15.80

45.9 46.5 0.6 4.94

BMU-25-023 1.95 2.45 0.5 29.93