Valve upgraded their Beta programm SteamVR, which is now compatible with Oculus Rift DK2. Meanwhile you can try the Big Picture mode with Half Life 2 and apart from that it also works with Windows and Mac OS X.
It was last Friday, that Valve announced this upgrade and it's now possible to play Half-Life 2, using Oculus Rift DK2 as your display. The issue these days is that Oculus Rift is still only a developers kit, meaning there are only very few select people actually able to try this new upgrade.
Should you actually be amongst the lucky folks, that have an Oculus Rift DK2 at hand, you might want to do some changes to the installation, since out of the box the gameplay won't be that smooth. On Reddit there is a guy explaining the different steps that need to be taken in order to make things run better:
Right click on Half-Life 2 (Beta) in your Steam library. Add to the launch options: -refresh 75 -freq 75. Go to the tab "Keyboard", choose "Advanced" and activate administrator mode. To make things available in the game press the "Tilde" key while being in game. In order verify the frame rates enter "net_graph1" and "net_graph0" and you'll see the frame rates.
With these settings it should be possible to enjoy reasonably smooth gameplay.
Source:
Le Comptoir du Hardware.