easy. take a circular saw and set depth to floor thickness (generally will be 5/8" OSB), and cut out the bad section. One tip is to screw down a straight edge onto the floor (like a yardstick) before you start cutting and use that as a guide edge for your saw as you go along. Reposition the straight edge the 3 remaining cuts so you can get a pretty perfect square cutout. Then toe nail or screw some 2x2 supports into your exisiting floor supports so you can have something to screw the new patch to. Cutout your patch, screw it the studs and finish off with floor patch or wood filler for any gaps. Then lay flooring over it