There are currently 2 ways the Oculus Rift interfaces with software. Natively, and with the help of a 3rd party program.
At present there are only two 3rd party programs to make older non-native games work with the Oculus rift. Vireio Perception and VorpX
Vireio Perception Cost: FREE! (Open Source)
The free and open-source 3D driver of the Oculus Rift. Vireio Perception will allow you to play your favorite games in stunning stereoscopic 3D custom tailored for head-mounted displays like the Oculus Rift. Take advantage of full 3 degrees of freedom head-tracking on supported titles and experience virtual reality like you never have before.
VorpX Virtual Reality 3D Driver Cost: Estimated $50 (Not released yet)
vorpX brings your favorite games to your virtual reality headset! Many DirectX9-11 titles are supported in true stereoscopic 3D. vorpX is more than just a 3D-driver, many features are specifically tailored towards VR, ensuring the best possible experience when playing your games on a VR headset.