I just saw this on twitter.
Rift Public Test Channel has our preview release which is fairly complete, we don't yet ship the .json manifest but you can create one and either point the XR_RUNTIME_JSON environment variable at it or set the HKLM\SOFTWARE\Khronos\OpenXR\1\ActiveRuntime reg key. The .json should look like (for x64):
"name": "Oculus OpenXR",
"library_path": "c:\\Program Files\\Oculus\\Support\\oculus-runtime\\LibOVRRT64_1.dll"
substitute LibOVRRT32_1.dll for the /reg:32 version.
I haven't tried it yet (I'm at work).
OpenXR is the cross platform AR/VR standard, allowing a VR app to run on any OpenXR compatible VR headset without changes.
Microsoft already supports OpenXR on WMR headsets. AFAIK Valve currently doesn't have support in SteamVR.