I am traveling from the United States to Thailand with a non-immigrant type O visa. Do I need an exit flight in order to enter Thailand even with a Visa?
Officially yes, in practice no.
Officially a Non-Imm O visa allows you a 90 day visit in Thailand, at which time you are supposed to depart. You can apply for an extension, which would give you 365 days from your initial entry, but approval is not a guaranteed, so the 90 days departure still technically applies. Thailand immigration officials almost never ask for proof of departure for folks with Non-Imm type visas (though they do have the right to ask).
However there is a greater chance that the airline flying you there might request you document your departure plans. I have been asked from time to time about my departure plans by the airline, as my reservations almost always exceed the official 90 days.
If you have a Non-Imm O-A, which is the "retirement visa" then there is no 90 day exit rule, rather a 90 days report to immigration to verify your presence type rule.