Selecting 'Zdepth' under 'Preview Pass type' doesn't seem to do anything to the rendered viewport and will display "Waiting for image..."
Tested w/ 2022.1.1 v27.12 stable
edit: I saw Lino's post about disabling "Use Shared Surface for Viewport" in the Octane Addon Preferences as it's a known bug. That looks to have fixed it!