Using this asset

[BastiaanOlij]

After installing this asset you need to add an ARVROrigin node to your scene and an ARVRCamera child node. Note that there are example scenes you can add into your project stored inside of this asset.

Then execute the following code in the _ready of your main scene:

var interface = ARVRServer.find_interface("Oculus")
if interface and interface.initialize():
	get_viewport().arvr = true

You can alternatively use a separate viewport if you want to free up the main display to render a 3rd camera or even a completely seperate view for a spectator. Make sure the ARVR property for this viewport is set to true and the world property is properly set. Also make sure you ARVROrigin node and child nodes are a child of this viewport.
Then change your startup code to:

var interface = ARVRServer.find_interface("Oculus")
if interface:
	interface.initialize()