I could of sworn OA was OGL, and MSI Afterburner even says that OA is OGL. I was speaking to a developer over at Evolve about why I couldn't get Overlay to work properly with OA, and after some testing, he came to the conclusion that OA was Direct3D.
To be specific as to how the conclusion was made, Evolve can hook into OGL and D3D9+ (some D3D8 and 7) games and when a hook attempt is made, Evolve will note this in a log. Evolve can auto-detect the renderer in use, but I tried a manual force of D3D and OGL. Evolve only actually reported success with the renderer in D3D. Also, in Evolve's defense, it detects the renderer right on every other game I tested, and also works fine in Quake III :p
Edit: Also, this is with the version of OA available from the downloads page (0.8.1) with the patch (0.8.5) on Windows 7 x64.
(also to anyone who has no idea what Evolve is, check out
this website. In short, it's a better program then Xfire and Raptr :p)