Today Microsoft have added support for OpenXR on Windows Mixed Reality. The compatibly app runs on Windows Mixed Reality PCs and HoloLens 2 (not the original HoloLens) and enables OpenXR support on the devices. SteamVR, Oculus and Daydream and also supporting OpenXR which is an open standard for VR and AR.
You can find the app in the Microsoft Store.
UPDATE: The Microsoft team behind OpenXR have posted some details about the support:
Hey I’m on the OpenXR team at Microsoft. We’re excited to see interest in OpenXR on Windows Mixed Reality! Right now the supported method to evaluate OpenXR 0.90 is described at https://aka.ms/openxr and is available through the Mixed Reality OpenXR Developer Preview app in the Microsoft Store. Just installing the application described in the article will not enable OpenXR support.
We’re working hard to help get the Khronos OpenXR 1.0 standard ratified and get support out to everyone as soon as possible!