@Serge ******
you are confusing things. .When you BUY an OA visa it's a year long multi-entry visa, that means you can enter exit all you want during the validity and you will get stamped in for a new year (or as long as your insurance is good for)
When the visa sticker (PDF) is getting close to expiring you exit/re-enter (with proof of new insurance) and you get stamped in for one more year.. THEN the visa itself expires and you're here on an entry stamp with NO re-entry permit. You don't get in on a single entry you get IN on the old visa and then when it runs out so does the re-entry permit that was attached to it
If you're going to leave during that "second year" (after the OA visa expired when you're here on the year entry stamp) you BUY a re-entry permit at the immigration office (1000baht single re-entry or 3800baht multiple re-entry) to get in and out on that stamp until it runs out