Can I Extend My 90-Day Non-O Visa in Thailand and What Paperwork is Required?

As you described, you have the Single Entry Non Immigrant O visa based on 50 years of age. On entry you get 90 days of stay and after 60 days you can apply at your local Immigration Office for the 1 year Extension of Stay based on being over 50 years of age.

Be aware that the used method of money in the bank is at least 2 months prior application date a minimum of 800.000 Thb in a Thai bank account on your name.

I don't know what a "pensioner 90 day non-o visa" is. I'm assuming this is a 90 day Non-O visa based on being 50 years old or older, and that you meet the minimum requirements for a year long extension of stay.

Is that correct?

Plus, if you are planning on getting that extension of stay, which immigration office are you planning on using?