Do I need a return flight ticket when entering Thailand with a Non-Immigration ‘O’ Visa?

You can buy a temporary verifiable onward ticket online for just a few pounds / dollars at Best Onward Ticket (others are available) for your own peace of mind. It's valid for a couple of days and then gets cancelled automatically. It's a real ticket with a PNR number which can be checked by airlines, immigration etc.

You don't need a return flight with a non O visa

Wording matters,it should read like this:

"I am entering Thailand on a 90-days single entry Non-Imm-O Retirement Visa, I shall get stamped in for a 90-days stay permit.

And will hopefully be converting this 90-days stay permit to a 1-year Extension of the Stay Permit based on being over 50/Retired "

NOTE: you do not convert to any “retirement visa”. The visa becomes invalid and “used” upon entering Thailand. Sadly Thai Immigration does not draw a difference between a stay permit and a visa. For them, in their inaccurate English, it is the same thing

And regarding your question: On a 90-days single entry Non-Imm-O Retirement Visa, you won’t be asked for proof of onward travel or a return ticket, by the airline.

If the check-in crew at the airport starts asking for this, demand to speak to the manager. As this Non-Imm visa-class is meant to get converted to a long term stay in Thailand, so there is no return ticket needed

You already have the “retirement” visa so you won’t be converting anything. You will be applying for a 1 year extension of stay.

Immigration won’t question it, it’s usually the airline when you check in at your departure airport

Hi John, when are you getting to Bangkok? My husband and I are going to be there on the same O Visa on November 27th. Trying to find somebody else that's got the same timeline around us so we can help each other out

If you have the non-O visa, there is no requirement for proof of onward travel out of Thailand. One way travel is fine.