If you don't get a Visa prior to coming On our recent vacation back to the states we did round trip startung from Bangkok. Upon coming back to Thailand we had to show the agent at check in our Thai visa in out passports . You could buy a one way to BĶK and also book a cheap flight out of Thailand to Laos from BKK then cancel it once you are in Thailand
I just flew Korean air/Delta to the US Delta flight was great. Korean air fell short. Neither of us was asked about visas. Maybe on our return to BKK. We have multi entry Non O visas