Is this not an official Thai Visa webpage? Just asking as it is where I applied for my OX visa 3 months ago and am now in Thailand on that visa. However I have 5 years before I need to extend it so have not looked to see if extensions can be done through it yet.
Are you a gambling man? Do you have a Plan B if you get to the airport and get refused entry? If so, go right ahead. If not maybe speak to the Thai Embassy in your home country and take their advice not the advice of random strangers like me on FB.