I was in the process to do a youtube video on my setup that I think works GREAT. But I'll quickly write out the steps I use.
Software,
1. OMV( of course)
2. SickRage
3. Some custom scripts.
SickRage(SR) I have this setup running on a external VPS server(because I have a few other friends using the application as well, but it does work on OMV directly. I have tested it out). I have subscribed to several tv shows. SR downloads the .torrent files, not the actual TV show, just the torrent file. I have a automated cron to build a RSS feed based on what .torrent files are in a directory. Back on my OMV at home. I have another script that grabs the files from the RSS feed places them into the transmission directory which in turn has them autodown, once they are downloaded I have another script that moves the completed downloaded TV shows into my PLEX directory.
Thats pretty much it in a nutshell. It's totally handoff setup. I don't need to visit websites to download torrents. It's pretty much like a PVR setup. Normally within 10-20mins after the show airs the torrents are pickup and sometimes within 30 mins I can be watching my fav tv shows. Great thing about having SR on the VPS(and I'm sure you can do this as well on OMV opening up a port) I can connect remotely, from work for example, and add a new tv show I may have just found out about. Add the new TV show, force it to download all the episodes , and by the time I get home they are all ready to watch on PLEX.
https://sickrage.github.io/