The OculusRift folder contains the source code for the C++ dll that talks to the Rift. It's based on the Oculus SDK.
The OculusRiftSample folder contains a MonoGame sample project that PInvokes into OculusRift.dll
In order to compile this, you first need to fix up the broken reference to MonoGame.
The OculusRiftSample_prebuilt folder contains the prebuilt binary version of the OculusRiftSample. If everything goes well you'll find yourself surrounded by pyramids. Use the mouse to move around.