It all depends. If YOU HAVE NOT GRADUATED, you can travel outside US, while your OPT is still being processed, for reentry, YOU NEED A VALID PASSPORT, and a F-1 VISA STAMP , and I-20,that is less than 1 ys old. F-1 student, who has filed for OPT to begin after graduation may travel, outside US, while OPT is pending. you need to present, EAD, of if your OPT is pending, the ORIGINAL I-765 receipt notice. For students, who have graduated, and had their OPT APPROVED, CANNOT TRAVEL OUT (well they can but ) and CAN NOT REENTER US , during the OPT process UNLESS THEY ARE ACTUALLY RETURNING FOR THE JOB ( meaning they actually have had a signed contract, or are out on a vacation to nepal or whatever, and has a written notice from their employer saying, he/she is employed till so and so date)...meaning you can only reenter US , TO RESUME YOUR JOB. NO JOB, NO REENTRY , NO OPT. BE CAREFULL.