I had friend who came back while he had 10 days left on VISA, So i believe if you have the VISA valid then you could. OPT, they won't start till you are fully done and certified by adviser that you are done completing your course which would be end of the semester before they even process your OPT until you have special cases like getting a job way before course completing (specially done with course but not the entire semester) and your adviser filed for early OPT...