There is an ongoing question regarding the need for a return/onward flight when getting a tourist visa to Thailand. I have posted several times it is not required, and I know I have no authority and so some people are going ahead and planning on getting one. Well, here is the link to Royal Thai Embassy in Seoul. When you get your visa and COE from them you are locked into flying on certain dates. You will see you are only required to purchase a one way ticket. Good luck.