How Can I Extend My Non-Immigrant O Visa While Living in Thailand?

Not to confuse you BUT here's another suggestion seeing as you've "decided to give Thailand a go and maybe retire here". ;)

You could go to the thai consulate in Savannakhet Lao (across the river from the thai province Mukdahan) and get a year-long, multi-entry Non-O visa based on marriage to a thai without the need to show proof of funds.

That visa is good for a year, is good for unlimited entries, each entry gets you stamped in for 90 days. When the 90 days runs out you go to the border, stamp out of thailand, stamp in&out of another country, turn around and stamp right back into thailand to get a new 90 day stamp.

On that year visa you can get 15 months of stay (in 90 day increments).

IF you're really needing time to decide IF you're gonna stay or not, that'd be the easiest and most trouble free way to do it

IF you got a 90 day single entry Non-O visa from the thai consulate in your country before you came here you have two choices to extend it

Get a 60 day visit thai family extension

OR

Get a year-long extension

They both cost the same amount (1900baht), but the year extension requires you meet the financial requirements of having 400K baht in a thai bank account in your name only for 2 months before you apply. Whereas the 60 day visit thai family extension does not have financial requirements.

you can't get a 3 months extension.:/