You can apply for an F-1 visa during your OPT year. Follow the regular procedures to apply for an F-1 visa (https://kysu.edu/international-affairs/prospective-students-3/prospective-students-prepare-for-your-travel-entry-to-us/) outside of the U.S. For the F-1 visa application, plan to provide the documents listed above, as well as proof of financial ability to stay in the U.S. and proof of continuing ties to your home country.
After the OPT approval start date, time spent outside the U.S. will count as unemployment against the 90-day limit. However, travel while employed either during a vacation authorized by an employer or as part of your employment will not count as unemployment. Please keep your primary DSO informed of any travel plans while on OPT that may affect your status.
If you have dependents in F-2 status who will travel without you, be sure they carry a photocopy of your EAD card and proof of your employment along with their updated F-2 I-20 that is properly signed for travel.
If your STEM OPT extension application is already approved, the travel document requirements are the same as those required during the post-completion OPT period.
If your STEM OPT extension application is pending with USCIS, it is okay to travel internationally. This is true even if your post-completion OPT EAD is already expired.
The preamble to the 24-month STEM OPT rule 81 FR 13103 par. 774 states:
“Students on STEM OPT extensions (including those whose application for a STEM OPT extension is pending) may travel abroad and seek reentry to the United States in F-1 status during the STEM OPT extension period if they have a valid F-1 visa that permits multiple entries and a current Form I-20 Certificate of Eligibility endorsed for reentry by the DSO within the last six months. The student’s status is determined by CBP upon admission to the United States or through a USCIS adjudication of a change-of-status petition.”
It is also possible to apply for a new F-1 visa during this time. The Foreign Affairs Manual, which instructs U.S. Department of State officials regarding visa issuance, states at 9 FAM 402.5-5(N)(6)(e)