How can I repair a hole in my subfloor?
Using a circular saw set to the thickness of the sub floor, cut a square between the center of the two nearest joists. The saw is held on an angle and slowly lowered to the floor, then moved to the cut lines. BE CAREFUL WHEN LOWERING THE SAW KEEP YOU LIMBS AWAY FROM THE BACK OF THE BLADE if you lower the blade to fast, it can wheel off backwards cutting a leg or so! Once you cut out your square a new piece of sub floor of the same thickness can be screwed in place in the joists. A 2 x 4" can be placed on each end as a "nailer" to join the old sub floor to the new piece.

See How to plunge cut