Want it Incorporate video into current audio only system,

^^^  What he said. The best way to connect video audio to a high end audio system is simply run the two channels into the high end system. Anything A/V (multi-channel) related will ruin the sound quality. Use a DAC as above if absolutely necessary. The results you can get with this "minimalist" approach can be impressive.  https://systems.audiogon.com/systems/8367

Yeah, if you’re truly just “watching TV” I’d just make whatever connection you need (as above) to what you have and be done with it.  If, however, you watch movies and would like to make that significantly more entertaining without changing your 2-channel setup at all there’s an easy and relatively inexpensive way to do that as well — just depends on what you want. 

    jokomo,
     Yes, it seems like you have everything you need, except for a cable or two, to do what you want  But I need answers to the following questions before I can describe exactly how to do it:

1.  What is the model number of your Bryston preamp?
2.  What source do you use for tv- antenna,cable, internet or satellite?
3.  What type of cables do you currently use to connect your current tv source to your Oppo 205 and tv?
    
Tim

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