Thanks for the replies so far! 
I'll try Game Capture then and also to check and change the Position/Size settings! I wanted to capture the windowed game because I only have one monitor and wanted to read the comments on my channel while playing.
@Neui, I'm actually still using the old OBS, so it's good to know that the newer version can't capture OpenGL. The old version captures the OpenGL window for me, but only partly. I'll play around with the settings suggested by TheMaster627. I'll let you know later if it worked out or not.
I'll try Game Capture then and also to check and change the Position/Size settings! I wanted to capture the windowed game because I only have one monitor and wanted to read the comments on my channel while playing.
@Neui, I'm actually still using the old OBS, so it's good to know that the newer version can't capture OpenGL. The old version captures the OpenGL window for me, but only partly. I'll play around with the settings suggested by TheMaster627. I'll let you know later if it worked out or not.
AMD Phenom II X2 555 @3.20 GHz, 4 GB DDR3 RAM, ASUS EAH5770 CUcore (ATI Radeon 5700 HD series, 1 GB), Windows 7 x64.