Can I apply for a retirement visa in Thailand while being visa-exempt, and how does the 90-day non-O visa work?

Unfortunately (for you 😕 ) you aren't gonna get away from showing proof of funds IF you're getting that 90 day non-O inside the country or even at a thai consulate in a nearby country.

You'd either need a letter from your embassy stating you get a minimum of 65K baht a month in income <- NOT available for UK, US, or Aussies) OR you'd need to bank 800K baht in a thai bank account in your name.

Now whether you can or can't get that visa in your country before you wing your way here will be up to the requirements posted on the website of the thai embassy you're applying for.

For retirement options, you can refer to the attachment as a very summarised comparison. Three from the left are popular options in this group.

Also, Multiple entry Non-O immigrant based on pensioner visa is available at only some embassies, not everywhere. This visa is valid for 1 year, giving you 90 days on every entry. But most likely you need to apply this in your home country. Check them about this.

Another option might be METV, which is valid for 6 months, 60 days every entry.

If you are in Thailand, maybe the <Non-O acquired inside Thailand option in the attached picture+following 1-year extension+re-entry permit for each stamp> might be reasonable.

As in the attachment, you can apply for Non-O retirement in Thailand if you meet the requirements. Those requirements are immigration office-specific, go to your local immigration office to get the handout of requirements. You may be able to check the website of your local immigration office.

Here is general information for in-country Non-O retirement.

/9.FOR-RETIREMENT-PURPOSES-50-YEARS-OLD-NON-O.pdf