Yes, you likely need proof of onward travel, even with income qualification for a Non-Immigrant "O" visa, as both airlines and immigration can require it. While immigration may not always check upon entry, airlines are responsible for ensuring you have the means to exit Thailand and will often check before you board. To meet this requirement, you can purchase a cheap, refundable, or changeable ticket, or even a bus ticket to a neighboring country.
Why proof of onward travel is needed
Airline requirement: Airlines are legally obligated to have proof that you can and will leave Thailand before you fly. They can be fined if they have to fly you back out if you're denied entry.
Immigration requirement: Thai immigration officials can and may ask for proof of your exit plan before you can enter.
Visa status: Non-Immigrant "O" visas are typically for short-term stays and require proof of exit within a specific timeframe, which is often 90 days.
How to get proof of onward travel
Buy a cheap ticket: Purchase the least expensive ticket available to a neighboring country, such as a bus ticket to Cambodia, says this Facebook post.
Purchase a changeable ticket: Buy a ticket with a change or cancellation policy to ensure you don't lose money if you need to change your plans.
Use a third-party service: Some services sell "onward travel" tickets that can be canceled after use, according to this Facebook post.
I want to do nonimmigrant o qualifying with income but I'm under 40 everything I've reads says I need onward travel.