In my AMD (Radeon) settings, you can set up a 'Profile' for the game you are using, to force the game to turn off AA (Antialiasing), its more a brute force option from the Radeon drivers to override the Game settings, or......
In the game, go to graphics settings > Antialiasing > Off. Most games will have an AA option. In my case, Path of Exile has 'Off', '4x', '8x'. I even looked in 'Victor Vran', and DIablo 3, they too have an AA option.
Just bear in mind, don't specifically look for 'AA', Look for 'Antialiasing'.
[edit] anyway, I have just verified with Diablo 3, AA has to be off for the game to show in OBS, so the problem (For me) is not localized to one game, it does it with Diablo 3, and Path of Exile.
So either its a Directx problem, Drivers (is it happening to Nvidia users too ?) or its OBS.