Sub floor Level for hardwood floor
That's OSB alright, very common as a subfloor. Sounds to me that your seams are peaked from expansion. Pay more attention to how flat the floor is than the bubble. I bet when the bubbles indicate level, your straight edge is not touching the floor in spots. How long is the level?
You may have to counter sink the fasteners and use a belt sander. You may have too much humidity or the panels were not installed correctly.
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