some question for Blend Render Compositor layer( OvrOverlay ) with Opaque & Transparent model

Yes, Oculus Home has 4 layers, which are rendered in the following order:

1. Cubemap background
2. Cylinder panel (tile grid)
3. Cursor
4. Eye buffers (controller geometry)

Each layer is blended on top of the previous ones using src.a * src.rgb + (1-src.a) * dst.rgb. So we set a=0 in each layer to "poke a hole" for the previously-rendered layer(s) to show through. In Unity, you can use the "Underlay Impostor" shader to poke a hole for OVROverlay underlays. Sorry this isn't in the documentation yet, we're updating it. For performance reasons, we can't easily change the blending formula, but you can change the compositing order by activating your OVROverlay instances in a different order. "Underlay" layers simply composite before the eye buffers and "overlay" layers composite after them.