@Mark *******
Your OA visa will have a "Visa must be used by" date. For example it is Dec 31 2023. Until then you can go and come as many times as you wish every time stamped in for one year. You do not need a re-entry permit by definition as it it is a multi entry visa. Then in this example If you fly to Thailand on New Years Eve you will be stamped in until Dec 30 2024. 「From then if you want to leave Thailand without getting a new visa or doing visa exempt you need a single or multi reentry permit to protect that last entry, because your original visa is no longer valid. There is no two year visa actually, that is just is a way of saying you got near two years stay power from one 1 year visa. Insurance is another issue.」