If you are refused entry and the airline brought you without meeting the entry requirements then they can be held responsible for taking you back so if you don't have some form of exit from Thailand booked within the terms of entry it is very likely that the airline may refuse boarding. I was refused boarding when I came over on a one way flight even though I had a non-O visa, I think the woman at the checkin desk either didn't see the visa stamp (I handed the passport to her opened on the visa stamp page) or didn't know about them. I was OK after explaining the stamp and that I was not coming back so all was OK. You don't have to have a flight back to your own country, you just have to have proof that you are leaving Thailand within the visa terms and I've seen posts that recommend just getting a train or bus ticket to Laos (before the end of the
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days) before traveling to Thailand and show them that as it's proof of leaving Thailand within the visa period if you have a problem, you don't even have to use it but it's cheaper than a flight.