I entered successfully on a one way ticket visa exempt. However it would appear that the airline at the point of departure are the governing factor. A round ticket to be safe is only a small cost addition, not much more than one way
Only problem I had was at check in, airline had confusion over visa on arrival vs visa exemption. If you ask embassy to provide verification your country is visa exemption this should prevent delays. Several people have had this problem
got all the way thru right up to final immigration then some bird noticed the dates different. Had to stay in naughty boys room for 20 mins till I showed other flight cancellation and replacement flight ticket. Then they let me off, big relief, no in ASQ
cheers, just got through check in after kerfuffle over difference between visa on arrival and visa exemption. They looked at all the documents and didnt comment the CoE is tomorrow's date so just the Thais to deal with on other end now.