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):
{
    "file_format_version": "1.0.0",
    "runtime":
    {
        "api_version": "1.0",
        "name": "Oculus OpenXR",
        "library_path": "c:\\Program Files\\Oculus\\Support\\oculus-runtime\\LibOVRRT64_1.dll"
    }
}
substitute LibOVRRT32_1.dll for the  /reg:32 version.