I'm not doing quite what you describe, and actually never got around to making any analog audio connection from the Oppo to the 990. I'm using digital audio output (coaxial in my case, although optical works just as well) for DVD viewing and for the occasional CD. DVD-Audio (which the Oppo will do over its 5.1 analog output) and SACD (which the Oppo won't do) are handled by my Yamaha DVD-S1500, which is connected to the 990 with 5.1 analog for those formats and optical digital for CD's.
The Oppo's analog section is pretty basic, so the 990's DAC's will probably be your best bet. That's why I'd suggest setting up the CD input to use the Oppo's optical input and set it up to use upsampling - that'll be your best bet for CD listening. On the other hand, if you want to listen to DVD-Audio through the Oppo, you will need to connect the Oppo's 5.1 analog output to the 990's 7.1 input. From what I've read, the Oppo is not the greatest DVD-A player on the block (there wasn't the budget for high quality DAC's, and in fact the Oppo originally didn't support DVD-Audio at all - it was added in a firmware update because they realized their hardware was capable of it). On the other hand, you've got the capability to play around with DVD-Audio by simply connecting half a dozen analog audio cables - if you have any DVD-A discs or are planning to pick some up, go ahead and connect the 5.1 output.