Go to immigration and request an extension that they will deny but still give you a 7 day extension to leave Thailand. 1900 baht but no overstay repercussions.
If you come visa exempt you'll be stamped in for 30 days extendable at immigration in Thailand for another 30 days. If your O-A is approved and you leave and re-enter you would be stamped in for a year. But you need to enter to get the new stamp