best way to convert hdmi out to coax or optical for pre amp input

Which model do you have? Are you trying to use the Oppo as a transport to a standalone DAC?  If so, it's my understanding that every Oppo has a optical out at else for CD audio. Are you talking about SACDs? I know that some Oppo players don’t output SACD audio to optical (licensing issues) and your only choice is to either use the built in DAC and output to RCA or you can output the audio to HDMI. With that said, why can't you just connect the RCA outputs directly to your pre-amp? The RCA outputs all the audio regardless of format. On a side note, I did heard some people having success buying a DAC that supports a HDMI input like the DACs made by Bryston.

Depending on your model, the DAC in your Oppo should be pretty good.  Have you A/B tested the optical out vs rca out with at else CDs? Is the Parasound that much better? Honestly, I'm not sure if it's worth your effort.  You might be better off buying a used DAC that supports hdmi inputs or selling your Oppo for a player that supports optical or coax out for all supported discs formats. Personally, I'm sick of switching cds so I ended up ripping everything I own and putting it onto a music server.  The funny thing is that I flip more records, more ever now.  I rarely use my Oppo player.