I don't have OMM Pioneer racks, but I have used Ortlieb panniers on other OMM racks.
I have had the panniers come loose on rough roads and singletrack.
There are a few solutions:
- I use 1 zip tie on each pannier to secure the bottom for longer dirt tours as it's 100% effective, cheap, easy and I don't need my panniers off the bike
- Ortlieb sells a variety of clips you can use to secure the lower part of your pannier...you can attach 2 clips on some panniers to help lock things down better
- a strap wrapped around the pannier and rack will keep it secure and allow you to remove it as needed from the rack
I for one will store that last tip of using a strap in the back of my mind, seems to me to be the easiest, most versatile approach for when you want to be able to easily take the bags off. (zip tie one too, although I dont do the offroad stuff you do, so it would be more of a "fix-it" thing)
thanks