@Graham *******
I will try to explain. Your O-A visa is valid for one year with multiple entries and one year only. If you max your stays out, it can give you a 2 year stay. You just have to time your last entry just before your visa expires. So even if you arrive on the last day, you get a stamp for another year. But the next day your visa becomes useless, it expires. You can stay here for another year according to your stamp but once you leave, you can not come back.
There’s the possibility to extend your O-A BEFORE it expires in Thailand as it has been explained above but you need to have the 800K in a thai bank. The extension, if granted, is a single entry Non-Immigrant O valid for one year and for that there is the possibility of getting a re-entry permit so you can get in and out.