Will onward tickets to Malaysia be acceptable for entry into Thailand with a 30-day visa exemption?

It's really up to how well trained the airline check in clerk is. If their trained in overseas travel and they see your on a one way, they will ask about onward travel after visa exception days.

Recently, I visited Thailand with a US passport..one way ticket..no issues at immigration...stayed 2weeks....bought outbound ticket ..

I flew from USA in November and was not asked by airline or anyone else about my onward ticket.

I did have one just in case

And I applied once I arrived in Thailand 🇹🇭

The only difference as I understand it is that when applying for non-o in Thailand you need the 800k in Thai bank at application time and it can be a challenge to get a bank to give you an account without the non-o and a residency certificate.

When applying in your own country you just need to show a bank account with the 800k (about $22kusd) in your home country Bank (or evidence of 65k income). You'll then have to get it into a Thai bank 60 days prior to applying for your first 1yr extension (30 days after your non-o is approved).

Everything else is the same.

I was told you had to apply for the non -o while still in United States? I am planning on moving in December 2024. Which is it> Can I come with just my passport, get an agent when I get there and apply for non-o there?

Nope. You'll need onward travel within 30days. I doubt Thai Immigration would be your problem. Boarding your flight will be the biggest problem

No

You might risk that your airline will ask for or require an onward ticket within the initial 30 days for visa exemption arrivals, regardless of if you’re entitled to do a 30 days local extension later.

A non-O doesn't require any onward ticket so you wouldn't have needed it for that.

Normally for proof of onward travel you will need to show a ticket out of Thailand within the number of days you'll receive when you arrive. If you're traveling without a visa that is 30 days.

Normally it will be your airline that asks if anyone does, so you should confirm with you airline what their requirement is for traveling without a visa to Thailand.

Also you should get the non-O before traveling to Thailand as it will be basically impossible for you to open the bank account you'll need in order to convert to the non-O visa in Thailand. So you'll be forced to pay an agent to open a bank account for you if you don't arrive with the non-O visa. It should be relatively easy to open a bank account yourself if you already have the non-O visa from the embassy.