I don't see an HDMI connector on the music-server gizmos you proposed. Would I have to install a PC in each room where I wanted to watch movies (and my classical audio/video concerts) for the HDMI video connection - in addition to the music-server for the line-level 5.1 audio connections?
Succinctly; no. (although with some software you actually could maintain master/slave type configs across multiple computers if you wanted to use that software specifically for everything related to browsing your library on all end-points)
Like has been mentioned before, view your Oppo units as end-points. Do whatever you want with them in the room, hook them up to a TV, to a surround system in that room, etc. The only thing concerned with hooking your media server up to the Oppo units would be running a network cable to each room and having everything connected to a Switch or Router as usual LAN configurations tend to go.
In Room A you could have your fancy unit that can send the video you wanna watch to the TV while outputting your audio over your multichannel audio system in that room.
Let's say room B is a kids room. Well you don't need anything fancy in there, maybe just a TV and a Roku to go with it, and you can browse the media server from that room, queue up a movie or some music or whatever is in the library.
Maybe Room C is your garage and you just have a simple stereo setup there.
Really the only thing concerned with getting the media from the server to your units is how you want to do it. You could just bitstream everything, which basically means the media server software doesn't touch it at all, and presents it to your Oppo in a format it should be able to read over the network, or HDMI or whatever. I'm not entirely sure if that's true for Video, but bitstreaming audio is a thing. Anything you do with video you can likely configure yourself (don't touch it, or; send it at really dumb high bitrate, whatever you want to do). Again that comes down to the software you choose.
But bottom line for a single server with multiple consumers. Wire everything up in a LAN and you are good to go for serving multiple locations around the home. It's really a matter of "I still have all the same units, I just don't have to switch optical discs out any more"