Under current Thai visa regulations, a foreign national is not permitted to fly on a one-way ticket to Thailand unless they are in possession of a valid visa which can be presented when checking in for the flight. Without this visa, the passenger may be refused boarding or may be requested to purchase an onward flight ticket.
Please also be aware that even if you are in possession of a valid visa from a Thai embassy/consulate, some flight operators may still be reluctant to allow you to board if you only have a one-way ticket. In this instance, it has nothing to do with the rules and regulations of the Thai authorities; it is down to the flight operator’s own policy. For this reason, if you intend to travel on a one-way ticket to Thailand always check with your carrier what their policy is and get it confirmed in writing.