No you do not get a Visa On Arrival. You can get a 30-day entry stamp which exempts you from any visa. This stamp can be extended once only for an additional 30 days
Chris Maple. If Cambodia is on the green list, you won't need to spend the 21 days there, but it sounds like a lot of trouble to go to when you can get a 1900 baht extension at an immigration office
Marty Pollard. That's how I used to do it with Australia. I'd always have a trip back home every 12-18 months, so sometimes I'd use the border in/out at Chong Mek to get the free extra time. By far the best retirement visa. In theory you need never transfer one cent to Thailand.