In all likelihood it is OSB oriented strand board, not the same as chip board and actually more resistant to water than plywood. If it is not particularly bad I would dry it out and use CPES Wood preservation, rot repair, and restoration using epoxy resin on boats and homes.
If it is serious enough you are going to have to replace a section. Cut out router out a lip on the under floor and replacement section to match, glue and screw. Were it me I would use an aluminum patch under the floor to reinforce the whole mess.