Can I book a return flight for up to a year or just one-way when entering Thailand on a 1-year retirement OA visa?

@Wyn *********

Thailand don't, some countries require a return flight 🛫 but not the airline, they will request because it's part of the entry requirements for them to take you to that country. Not Thailand even on a tourist visa it's not required. You may be booked on another airline so it's none of their business unless it's an entry requirement.

@Wyn *********

not on long term visas. Only on tourist or visa exempt.