What should I do before leaving Thailand with a one-year OA multiple entry visa and returning later?

As an update I chatted with the OP...

Thankfully he has a one year policy from ACS and LMG which together meets the requirements of the OA visa and entry and reentry into Thailand.

He will extend it after he reenters for one added year and take proof of the LMG policy extended one full year to Thai Immigration to update/extend his permission to stay date to match his arrival date plus one year.

As a relevant aside, you will be stamped in until the expiration date of your compulsory health insurance, to a maximum of 1 year. (That’s not the additional Covid insurance currently required, but the health insurance coverage for minimum of 0 inpatient and outpatient.

When an O-A Visa holder exits Thailand within the first year and then reenters Thailand the O-A Visa is automatically stamped in for 12 more months. This has nothing to do with an Extension of Stay... different kind of deal.

@Daniel ****

, so as others' comment, check your visa sticker AND your current entry stamp (permitted until stamp).

IF valid date of the VISA STICKER is later than September, you just leave and come back (regarding to visa-matter). But if your valid until date on VISA STICKER is before September and permitted until date of your current entry stamp is later than that, you can get re-entry stamp BEFORE you leave, and you will be stamped until same date to current stamp upon entry in September.

Be careful not mix those dates.

Are you in the first year of the OA visa?

If so you have a multiple entry visa.

But will need to meet the entry requirements of the Thai Embassy... Obtaining permission to enter, insurance and quarantine too.