Traveling while your OPT is pending is not recommended. If there is an urgent reason why you need to leave the U.S., please discuss this with Immigration Services.
OPT is essentially an extension of your of your F-1 status. Therefore, you still need to obtain the appropriate signature from Immigration Services (or your F-1 sponsor if you are a UMMS employee working on a period of OPT based on your completion of a degree from a U.S. University) on page 3 of your SEVIS I-20 before traveling if the signature on your I-20 is more than 6 months old. Make sure that when traveling, you have your Form I-20, Employment Authorization Document (EAD), valid passport, valid F-1 visa stamp and job offer / employment verification letter.
If your F-1 visa stamp has expired and you need to apply for a new visa stamp, please keep in mind that this can be risky since you are no longer enrolled in your degree program. In addition, travel outside of the U.S. can also be risky if you cannot show that you have a job or job offer. Contact Immigration Services before your departure if you would like to discuss this in further detail.