rebbi - With HDCD, the decoder is in the DAC. So, if you use the Oppo analog outputs, you will get the full benefit of HDCD encoding. If you use digital out, with an external DAC, you will lose some of the benefits of HDCD encoding, unless your external DAC can decode HDCDs. But most people who know about these things say that even with non-HDCD decoding, HDCDs can sound better than standard Red Book CDs, all else being equal, of course. Something to do with the 20 bit depth of HDCD vs. the 16 bit depth spec of Red Book, IIRC.
One thing to note is that many HDCDs are not labelled as such, but playing them in a player with HDCD decoding and an HDCD indicator lamp, like the Oppo 93, will reveal that.