Sriram,
In addition to what JohnK and Sir Quack have suggested, check the Oppo's menu to see if it has a "disable HDMI audio" setting. You can use that if you do NOT want the audio going to your TV's speakers. I don't have the same model Oppo as you do, but mine has the above audio option in the audio setup menu.
Once you run either a digital coaxial audio (you can use any old coax RCA cable) or an optical digital cable from the Oppo's digital audio output, your Sherwood should decode any DVD's Dolby Digital or dts 5.1-channel soundtrack and send it to your Axiom speakers.
When the Sherwood has automatically locked onto the Dolby Digital or dts data stream, it should display that info in its front panel or on-screen display. "Dolby Digital" or "dts". It might even say "5.1".
If you want to play SACD or DVD-Audo hi-resolution audio discs, you'd need to connect the Oppo's multi-channel analog outputs to the multi-channel analog inputs on the Sherwood. Those would be RCA jacks labeled front Left, Right Center, Left Surround, Right Surround, Subwoofer (or similar labels).
Regards,
