Hardwood flooring sub floor problem
OSB is prone to expansion if it gets moist, which is I suspect what happened. It is supposed to be installed with a 1/8 inch gap between sheets to reduce the chance of upraising if the subfloor gets damp, or absorbs moisture from the air. I am guessing the subfloor was installed without a gap, and while you can certainly sand the joint down, there is a chance the subfloor will raise up at the same location, causing an uneven floor.
I suggest you cut a 1/8 inch gap into the subfloor between the sheets, if there is no gap. Use a circular saw with a track, and use a carbide nail cutting blade. Set the depth to the thickness of the subfloor, and cut the grooves in. Once cut, you can sand smooth and you will most likely have permanently cured the problem.