^ It's simple. Buy a Y adapter cable -- Radio Shack or online at Monoprice.com -- with a stereo RCA plug pair at one end and a stereo headphones socket at the other end. Costs a few bucks.
Plug the stereo RCA pair into the LF/RF Analog output jacks of the OPPO. Plug your headphones into the other end.
In the OPPO, set the Analog Output Volume to Variable. Use the +/- Volume buttons at the top of the OPPO remote to control Volume. (That won't change the Digital audio Volume to your TV.)
That's it for stereo content. If you are also listening via headphones to Multi-channel content, set Speaker Configuration > Down Mix to Stereo in the OPPO. Again, that won't change the digital audio to your TV.
Should work fine even with no amp in between if your headphones don't require a lot of output level.