Is it buckled or cupped? If it's cupped, you might be able to sand it down, I'm assuming this is solid, not engineered, and there is a good chance it will cup again. If it is buckled, you may have to replace the subfloor as it most likely turned loose. Did you nail it or use cleats? What did the wood mill recommend? I would personally not install wood on an OSB subfloor in an area where there is extreme moisture and humidity swings.