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Osisko Development Completes 13,000-METER Infill Drilling Program at Cariboo Gold Project and Provides Update on Ongoing Lowhee Underground Drilling; Intercepts Include 44.68 g/t Gold over 1.5 Meters from 27.9 Meters Downhole, Including 133.62 g/t Gold over 0.5 Meters from 28.4 Meters Downhole

Drill Results Resource Estimates Exploration Programs

ODV NYSE TSXV News Release

OSISKO DEVELOPMENT COMPLETES 13,000-METER INFILL DRILLING PROGRAM AT CARIBOO GOLD PROJECT AND PROVIDES UPDATE ON ONGOING LOWHEE UNDERGROUND DRILLING; INTERCEPTS INCLUDE 44.68 g/t GOLD OVER 1.5 METERS FROM 27.9 METERS DOWNHOLE, INCLUDING 133.62 g/t GOLD OVER 0.5 METERS FROM 28.4 METERS DOWNHOLE

HIGHLIGHTS

13,684 m of grid infill drilling completed (2,659 m new results) on 10-m drill spacing in 142 drill

holes from August 2025 representing the entirety of the planned Appian infill program

 Highlight intercepts include: 44.68 g/t Au over 1.5 m from 27.9 m downhole (including 133.62 g/t Au over 0.5 m from 28.4 m downhole), and 25.78 g/t Au over 2.5 m from 41.3 m downhole (including 103.47 g/t Au over 0.5 m from 42.8 m downhole, and 12.51 g/t Au over 1.0 m from 41.8 m downhole)  2,995 m of additional infill and near mine exploration drilling completed as part of ongoing

Lowhee zone underground diamond drilling

 Highlight intercepts include: 31.64 g/t Au over 2.0 m from 50.0 m downhole (including 125.24 g/t over 0.5 m from 50.5 m downhole), and 11.21 g/t Au over 4.9 m from 63.3 m downhole (including 58.19 g/t Au over 0.7 m from 65.0 m downhole, and 17.57 g/t Au over 0.7 m from 63.3 m downhole).

Results to date continue to contribute to a more detailed understanding of spatial controls and

local variability within Lowhee Zone and give positive indications for near mine potential

Montreal, Québec, June 9, 2026 – Osisko Development Corp. (NYSE: ODV, TSXV: ODV) ("Osisko Development" or the "Company") is pleased to announce new infill drilling results from its completed 13,000-meter program on 10 meter drill spacing, as well as new results from additional infill and near mine exploration activities in the Lowhee Zone of the Company's permitted, 100%-owned Cariboo Gold Project (the "Project"), located in central British Columbia ("B.C."), Canada. Three of the six drill hole fans reported herein comprise the final 2,659 meters ("m") of the 13,000 m underground infill program, bringing the final tally of drilling with full results to 13,684 m. The remaining three fans comprise an additional 2,995 m of infill and near mine exploration drilling at the L1290-ORE-000 and L1260-ORE- 002 levels. Reconciliation work is underway on an updated localized block model, with final results anticipated in the second quarter of 2026.

Chris Lodder, President, stated, "The completion of the tighter infill drill spacing gives us a better understanding of vein corridor spatial geometries and local variability specific to this part of the Lowhee deposit, and gives us confidence going forward to help refine infill drill requirements, production designs and sequencing in the Lowhee deposit of the Cariboo Gold Project. The additional infill and near mine exploration intercepts reinforce the importance of continued drilling in underexplored zones."

DRILL ASSAY HIGHLIGHTS

This news release includes assays from fifty-five (55) underground infill and near mine exploration HQ diamond drill ("DD") holes (63.5-millimeter diameter) totaling 5,654 m with depths ranging from 76.5 to 138 m (see Table 1 and Figure 2). Select photon and fire assay highlights include:

  • 44.68 grams per tonne ("g/t") gold ("Au") over 1.5 m in BMU-25-176 from 27.9

downhole, including:

  • 133.62 g/t Au over 0.5 m from 28.4 m downhole
  • 25.78 g/t Au over 2.5 m in BMU-26-055 from 41.3 m downhole, including:
  • 103.47 g/t Au over 0.5 m from 42.8 downhole, and
  • 12.51 g/t Au over 1.0 m from 41.8 downhole

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  • 31.64 g/t Au over 2 m in BMU-26-034 from 50.0 m downhole, including:
    • 125.24 g/t Au over 0.5 m from 50.5 m downhole, and
  • 11.21 g/t Au over 4.9 m in BMU-26-031 from 63.3 m downhole, including:
    • 58.19 g/t Au over 0.7 m from 65.0 m downhole, and
    • 17.57 g/t Au over 0.7 m from 63.3 m downhole
  • 21.84 g/t Au over 2.5 m in BMU-26-008 from 79.7 m downhole, including:
    • 91.3 g/t Au over 0.5 m from 79.7 m downhole, and
    • 6.79 g/t Au over 0.5 m from 81.2 m downhole, and
    • 5.97 g/t Au over 0.5 m from 80.7 m downhole, and
    • 2.66 g/t Au over 0.5 m from 80.2 m downhole, and
    • 2.47 g/t Au over 0.5 m from 81.7 m downhole
  • 91.99 g/t Au over 0.5 m in BMU-26-013 from 73.75 m downhole
  • 18.59 g/t Au over 2.65 m in BMU-26-006 from 43.35 m downhole, including:
    • 48.93 g/t Au over 1.0 m from 43.85 m downhole
  • 4.76 g/t Au over 6.05 m in BMU-26-037 from 85.0 m downhole, including:
    • 44.79 g/t Au over 0.5 m from 88.5 m downhole, and
    • 6.13 g/t Au over 0.5 m from 88.0 m downhole, and
    • 4.02 g/t Au over 0.5 m from 87.0 m downhole
  • 3.28 g/t Au over 7.3 m in BMU-25-179 from 81.7 m downhole, including:
    • 12.73 g/t Au over 0.95 m from 84.85 m downhole, and
    • 9.68 g/t Au over 0.9 m from 85.8 m downhole
  • 46.37 g/t Au over 0.5 m in BMU-26-022 from 14.0 m downhole

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

DISCUSSION OF RESULTS

  • Figures 4-6 (1260 level): Cut-off assay composites completing the planned 13,000 m systematic grid infill program. These last three fans were testing areas peripheral to reserve stopes, and outside the modeled vein corridors. Intercepts received to date suggest possible extensions of vein corridors, with potential for resource conversion and upside mineralization.

Figures 7-8 (1290 Level): Cut-off assay composites from additional infill drilling at the 1290 • levels show a degree of spatial correlation with the modelled reserve stopes with intercepts outside these areas suggesting potential for resource conversion and upside mineralization.

  • Figure 9 (1260 Level): Cut-off assay composites from infill and near mine exploration drilling to the southeast at the 1260 level include several new intercepts outside the current vein corridors and resource footprint.
  • All intercepts will be incorporated in the planned remodelling and mineral resource calculation process to determine their implications on an 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.

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

Figure 2: Location and overview of ongoing Lowhee Zone underground definition infill and near mine exploration diamond drilling

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

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Figure 4: L1260-ORE-003 Lowhee Zone grid infill select underground drill assay highlights (this release) with

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Figure 5: L1260-ORE-003 Lowhee Zone grid infill select underground drill assay highlights (this release) with

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Figure 6: L1260-ORE-003 Lowhee Zone grid infill select underground drill assay highlights (this release) with

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Figure 7: L1290-ORE-000 Lowhee Zone infill select underground drill assay highlights (this release) with

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Figure 8: L1290-ORE-000 Lowhee Zone infill select underground drill assay highlights (this release) with

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Figure 9: L1260-ORE-002 Lowhee Zone infill and near mine exploration select underground drill assay highlights (this release) with previously released surface and underground diamond drilling results in cross section by fan. Results from immediately adjacent fans within slice omitted for visual clarity (20 m slice looking 051°).

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Table 1: Length weighted assay composites and individual samples>=1.8 g/t for Lowhee Zone underground DD.

Drillhole IDFrom (m)To (m)Length (m)Au (g/t)Est. True
Width (m)
BMU-25-167 14.8 15.8 1 2.11 0.89 Including 14.8 15.3 0.5 3.23 49.4 50.4 1 2.23 0.77 Including 49.4 49.9 0.5 3.84
BMU-25-170 4 4.5 0.5 10.57 0.43 13.55 14.55 1 4.00 0.82 Including 14.05 14.55 0.5 7.86 41.5 42 0.5 5.45 0.43 58.5 59.5 1 2.84 0.91 Including 58.5 59 0.5 5.40 80.5 82 1.5 3.18 1.36 Including 80.5 81.5 1 4.69
BMU-25-173 17 17.5 0.5 2.53 0.43
BMU-25-176 3 3.55 0.55 2.46 0.28 27.9 29.4 1.5 44.68 1.36 Including 28.4 28.9 0.5 133.62
BMU-25-179 81.7 89 7.3 3.28 6.96 Including 84.85 85.8 0.95 12.73 and 85.8 86.7 0.9 9.68 90.5 92.4 1.9 2.01 1.87 Including 90.5 91 0.5 7.02
BMU-26-001 44.7 46.2 1.5 1.86 1.36 Including 45.2 45.7 0.5 4.78
BMU-26-002 31.7 34.2 2.5 7.08 2.27 Including 32.2 32.7 0.5 32.31 and 33.2 33.7 0.5 2.30 48.2 50.3 2.1 10.09 1.90 Including 48.9 49.8 0.9 23.31 65.8 68.1 2.3 9.52 2.08 Including 65.8 66.4 0.6 5.02 and 66.4 67.1 0.7 26.38 74 78.7 4.7 1.96 4.26 Including 78.2 78.7 0.5 16.64
BMU-26-003 1.5 3.1 1.6 2.47 1.45 Including 2 2.6 0.6 6.09 35.5 39 3.5 2.12 3.45 Including 35.5 36 0.5 12.60 73.35 75.85 2.5 7.30 2.46 Including 73.85 74.6 0.75 5.39 and 74.6 75.35 0.75 18.36
BMU-26-004 58.5 59 0.5 3.11 0.41 66.5 67 0.5 2.85 0.43
BMU-26-005 1 3.25 2.25 4.70 1.59 Including 1 1.5 0.5 12.10 and 2.75 3.25 0.5 8.91 14 16 2 5.28 1.41 Including 14 15 1 10.40 17.5 18.5 1 12.24 0.77 Including 18 18.5 0.5 23.47 30.9 31.9 1 7.13 0.82 Including 30.9 31.4 0.5 14.12 37 38 1 2.34 0.71 Including 37.5 38 0.5 4.52 40 41.5 1.5 4.13 0.86 Including 40 40.5 0.5 10.91 58 64 6 2.10 4.12 Including 62 62.5 0.5 10.88 and 62.5 63 0.5 10.99 76.25 78.8 2.55 5.92 1.18 Including 76.75 77.25 0.5 23.10 and 78.3 78.8 0.5 6.86 80.05 81.1 1.05 4.98 0.60 Including 80.05 80.55 0.5 7.50 and 80.55 81.1 0.55 2.68

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Drillhole IDFrom (m)To (m)Length (m)Au (g/t)Est. True
Width (m)
90 91 1 2.18 0.64 Including 90 90.5 0.5 4.22 104.5 105 0.5 2.97 0.32 117.05 119.05 2 6.06 1.15 Including 117.55 118.05 0.5 4.97 and 118.05 118.55 0.5 17.27 128 129 1 1.96 0.71 137.4 138 0.6 9.22 0.42
BMU-26-006 2.95 3.95 1 12.53 0.71 Including 3.45 3.95 0.5 24.91 33.9 35.9 2 3.79 1.64 Including 34.9 35.4 0.5 11.52 and 35.4 35.9 0.5 2.63 43.35 46 2.65 18.59 1.70 Including 43.85 44.85 1 48.93 55.85 56.35 0.5 2.28 0.32
BMU-26-007 No Significant Assays
BMU-26-008 5.5 6 0.5 2.74 0.41 11.9 12.4 0.5 1.90 0.41 23 23.5 0.5 4.72 0.38 25 25.5 0.5 3.09 0.50 29 29.5 0.5 3.04 0.50 79.7 82.2 2.5 21.84 1.92 Including 79.7 80.2 0.5 91.30 and 80.2 80.7 0.5 2.66 and 80.7 81.2 0.5 5.97 and 81.2 81.7 0.5 6.79 and 81.7 82.2 0.5 2.47 98.1 99.1 1 4.33 0.64 Including 98.1 98.6 0.5 2.79 and 98.6 99.1 0.5 5.87
BMU-26-009 No Significant Assays
BMU-26-010 47.5 52.45 4.95 1.88 4.39 Including 47.5 48 0.5 6.10 and 48.5 49 0.5 7.42 and 51.45 51.95 0.5 2.77 63.45 65.6 2.15 2.53 2.08 Including 63.45 64 0.55 8.41 69 69.5 0.5 2.58 0.43 111.95 115 3.05 2.10 2.87 Including 111.95 112.5 0.55 8.80
BMU-26-011 31.9 32.4 0.5 16.65 0.41 35 36 1 2.61 0.91 Including 35 35.5 0.5 3.61 37.5 38 0.5 4.43 0.50 49 52 3 5.81 2.82 Including 49 49.5 0.5 8.54 and 51 51.5 0.5 24.00 55.2 55.7 0.5 7.31 0.45 66.45 69 2.55 6.93 2.02 Including 66.45 66.95 0.5 30.60 and 66.95 67.45 0.5 4.21 73 73.5 0.5 2.29 0.45 79 79.5 0.5 2.33 0.41
BMU-26-012 15.5 16 0.5 3.66 0.38
BMU-26-013 46 46.5 0.5 2.45 0.38 62 66.5 4.5 3.09 4.03 Including 63.25 63.75 0.5 2.15 and 63.75 64.25 0.5 6.20 and 64.25 64.75 0.5 17.14 68 71.5 3.5 2.05 3.29 Including 70.5 71 0.5 12.14 73.75 74.25 0.5 91.99 0.45
BMU-26-014 2 2.5 0.5 8.09 0.41 4.5 5 0.5 3.22 0.49 35 39.2 4.2 4.24 3.22

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