Can I apply for a 90-day Non-O extension at the Thai Embassy in Phnom Penh, Cambodia?

You must ask all these visa expert's thy now all better than imagrasion

you mixed the names! In Cambodia, you apply for the "visa". Back in Thailand on Immigration, you apply for the "1-year Extension of stay permit" (which is NOT a "visa"). You must be able to show the financial proof for both the visa application and the 12- month extended stay permit application

That will work but you've got the names the wrong way around. In PP you get the Non-O Visa giving you a 90 day entry then when you meet the requirements you apply for the 12 month extension. Check on the Thai Embassy website for PP to see the requirements for the initial Visa.

You can not apply for the NON O retirement visa at a Thai Embassy abroad (only in your home country).

If you are married to a Thai citizen you can apply for a NON O visa based on your marriage at a Thai Embassy abroad and in your home country.

You're a little mixed up, or more likely using the wrong terminology.

You cannot get an extension from a Thai embassy, as extensions are only done at an immigration office in Thailand.

You can get a non-O visa at an embassy, and it will be valid for 90 days, and give you permission to stay in Thailand for 90 days once you enter.

You may also be able to apply to convert to a non-O visa inside Thailand, but you would need to have the bank account already, and having the non-O visa could potentially help you obtain the bank account.

You would need to check the website for the embassy you want to use and see what their requirements are for the visa you want, and also check to see if they require an appointment.

Once you're in Thailand you would want to open a bank account and transfer the money quickly as you need the money in the account for 2 full months before you apply for your extension, giving you a window of only 30 days to get it done within your 90 day permission to stay.