Can I maintain my "O" visa by keeping 800,000 THB in my Thai bank account without touching it?

Yes. That’s what most are doing.

You need to keep 400k all year and 800k for 5 months only. But I don’t risk to forget it and loose the extension!

Can you meet the income requirements? I thought if qualified income no bank deposit, or lower amount ok. 400 000 baht.

It's fairly straight forward if you have the funds deposited in the bank. We've been here three years - always keep our balance above 400K baht each (Joint account so 800,000 baht) making sure we have 0 baht each (1,600,000 baht) 3 months prior to renewal and 2 months after. Supply bank book, bank statement, bank certification of account letter, lease / TM30 with the ID of the land owner, passport photos and passport and proof of 90 day check in being compliant (at least thats what Hua Hin immigration required 2 weeks ago) and pay 1900 baht. (2000 baht if it is first time I think? ) Took us about an hour from walking in to walking out with our stamp.

You don't actually renew the visa. Once the 12-months is up it's done. You simply apply for extensions of stay

Immigration love applications like yours.

I do that. I keep my 800,000 + thb in one bank account and use a second bank account for my day to day use. Works well but Thai bank interest is crap low.

Yes that's the safest path.

Never an issue that way.

I think most people do that. Just leave the 800k untouched. Very easy for immigration to see and understand

Many that can afford to do so indeed have separate accounts. One for immigration where they leave the amount untouched and use another account for spending. No issue with that other than the pretty much lack of interest on the 800k. Lucky if you get 1%