Flatheaded Borer in Firewood: What You Found

The odd, flattened larva tunneling in your fallen oak is a flatheaded borer—larva of a metallic wood-boring beetle. Beetles lay eggs on stressed trees; larvae feed under the bark, potentially girdling the trunk. They continue developing in dead wood, which is why you found them in firewood.

Unless your other trees show stress (thinning canopy, dieback), no treatment is needed. Healthy trees resist borers.