I am planning to lay 3 1/4" maple. The subfloor is 5/8" OSB over which I plan to install 3/8" plywood. The floor is relatively flat, as the house is 8-9 years old, however, one joist is quite crowned which has created a small peak. Additionally, the floor slopes off on one side of this joist all the way to the wall (arround 18" to the wall). The drop from the peak is significant - 3/8" I would guess. It would be difficult to lift up the OSB as it runs underneath the wall, but I suppose I could cut out a portion. It is advisable to sand down the OSB peak and then use some plywood and / or levelling compound to fill in the valley, or should I lift the OSB and plane the crowned joist? Also, what length of nails / staples would you recommend to fasten the 3/8" ply? thanks.

Lift the OSB, and fix it from underneath.

I thinkFloorguy is on the right path and giving good advice on this issue.

I just wonder what can be seen from underneath the house and why is that one joist a problem.

I would hazard a guess that it is a high joist situation instead of crowning. High joist means it is taller than normal which requires planing down. I have seen more high joist than crowns. Last job I repaired was 2x10's that are supposed to be 9 1/4" but they varied from 9 1/8" all way up to 9 9/16" in width.

My friend had a crowned joist, the contractor cut the joist at the peak of the crown, then was able to pull it down with some sort of configuration. Then another 2x10 x 32" (approx. 16" on each side of the cut) and secured it with four or six lag bolts. Seemed to work good... Now I should mention this contractor was fired for other reasons but I was impressed by this solution... I'm curious to hear what the more experienced flooring professionals thing.

That could work but I would find that no easier than removing some subflooring, planing down the high spots on the joist and re-installing the subfloor.

Gary has a great suggestion. Simple is always best.