Want it Incorporate video into current audio only system,

Your Oppo clearly has a better DAC than any TV.  For that reason I would use either digital or hdmi from the TV to the Oppo.  The Oppo is already connected to your pre, so you are good to go.  The tv should output all smart sources to the Oppo, including cable.  Just check "external speakers" on your tv set up.

I also have the Oppo 205 and I have Xfinity cable tv. I connect the 4K cable box to the 205's input using an HDMI cable. Then I use another HDMI cable to connect the 205's #1 video output, which is the output from the 205's internal video processor, to the TV.
      I then connect the 205's main left + right balanced xlr audio outputs to my preamp's left + right balanced xlr home theater (HT) inputs using a pair of balanced xlr cables. If your preamp only has unbalanced rca inputs, you can also use a pair of standard rca cables from the 205 to the preamp. The 205 has both xlr and rca left + right analog audio outputs.    
     If you just want to hear tv sound through your main speakers, you need to next go into the 205's Setup Menu and configure the settings on the Audio Output Setup and Audio Processing sections. Once this is completed, you'll need to turn the volume completely down on your tv,select the proper input on the 205 and your preamp and use the 205's remote to control the volume. Following the above steps will result in obtaining the most basic TV and HT audio from your system.
      If you'd like, however, you can utilize the full capabilities of the 205 and setup a complete 5.1 or 7.1 HT audio surround system. I utilize my 205 in a combination music and 5.1 HT audio surround sound system. Creating a full 5.1 audio surround system requires the addition of a center channel speaker, a pair of rear surround channel speakers and 1 or more subwoofers, along with the separate amp channels to drive them. A 7.1 audio surround system requires the addition of side left and right channel speakers and the separate amp or amps to drive them. The main speakers double as left + right stereo channels for music and left + right front channels for HT. These full audio surround systems are very impressive and enjoyable for HT listening to most with the noted exception of millercarbon.
     If you want an idea of what these type of combo stereo music and HT surround systems look like you can look at my profile and system pics.  

Best wishes,
    Tim

Thanks noble100 for the clear and concise answer.  That will be the way I will go. I didn't mention before but I have some used b&w surrounds and sub I picked up for a song a few years ago.  If I did want to X with 5.1, any suggestions for amps/control.  Integrated?

Hello jokomo,

     If you want to try 5.1 surround sound, you just would need 3 channels of amplification, one for the center channel and two for the pair of rear surround channels.  I'd suggest using a 3 channel class D amp for simplicity, reasonable price and good performance. I'd recommend just googling "3 channel class D amp" or "3 channel class D amp reviews" to get an idea of what's available.   I know that D-Sonic are high quality amps at reasonable prices. 
      I'm assuming your sub has its own built-in amp.  You would just connect the outputs on the back of your 205 to the 3 ch amp using standard rca cables and the amp is connected to the speakers using regular speaker wire.  To connect the sub to the 205, you'd just need a single standard rca cable.
     You also have the option of using what's called a "phantom center channel".  This uses your l+r main speakers to create the perception of a center channel without actually using a discrete center channel speaker.  This may work well for your purposes. You can read about this, and how to configure it on your 205, in your Oppo 205 manual.

Best wishes,
   Tim