Traveling while your application is pending should be undertaken with caution and avoided if possible.
Here are some possible travel scenarios:
Traveling before your completion date and while you have a pending OPT application. Returning to the US before your completion date is permitted because you are still a student even when your application is pending.
Traveling after your completion date and while your OPT application is pending. This is not recommended because once you complete your program you will be expected to have your EAD card in hand to re-enter the United States.
USCIS may send you an RFE (Request for Evidence) while you are away, so you would want to make sure you have provided the correct U.S. address both to your DSO/school and on the application and would be able to send in any requested documents.
Also, if USCIS approves your Post-OPT application, you will be expected to have your EAD card in hand to re-enter the United States. Like an RFE, USCIS can only send the EAD card to your U.S. address.
Travel signatures while on Post-OPT are only valid for six months.
For more information on travel, please read through Travel While on F-1 OPT.