Do I need an onward flight ticket when arriving in Thailand on a one-way ticket with a US passport?

The user is inquiring whether an onward flight ticket is necessary when arriving in Thailand on a one-way ticket with a US passport, as they plan to apply for a Tourist Visa and a NON-O visa once there. The comments highlight that while immigration may not require it, airlines often do, and there have been instances where travelers were denied boarding without a return or onward ticket. It is also suggested that obtaining the non-O visa beforehand could eliminate the need for an onward ticket.