It doesn't bypass the Onkyo volume control, if you turn the dial you will change the volume. However you can setup the Onkyo to always power on at any volume you want. I used 0dB which gave me pretty even trims compared to the Denon's internal amps. It's really easy if you have a working display, something the OP didn't have.
As far as your Oppo I think it would work great for that. Try it, you have nothing to lose right? I'm assuming you already have an 805.