Can I apply for a NON-O visa in Thailand while working part-time for a US employer?

You’ll need 800k in the bank for 2 months before you’re extension some offices want it 2 months before you apply for the non -o your monthly won’t Nader until your 2nd extension

You do not want to show any current income. Not only would it require a work permit but it would be taxable. Online work in the US would have to be paid and taxed in the US. Do not give up your American bank account.

The O visa is work prohibited. If you work it will be illegally. If you want to work legally look at the LTR visa.

I'm sure Tod Daniels will respond in more detail, but you need 800,000 baht the first year seasoned two months. Then you can use baht method.

you will not be able to use the monthly income method by international transfer for proof of funds to get an in country Non-O OR the year extension.

As you are from the US (a country that no longer issues the "affidavit of income from abroad" notary at the embassy here) you have to transfer in a minimum of 65K baht a month, each month, every month for the previous 12 months BEFORE you can apply.

That's the rule using monthly income method for countries who can't get the affidavit from their embassy.

You would have to transfer 800K baht into a thai bank account in your name only and use the banked money method to get the initial 90 day Non-O and the year extension after that. You could also begin transferring in 65K baht a month and use that NEXT year for the second yearly extension after you have 12 months of incoming transfers.