I wrote a couple of config files that make OPM stable with Principia. You can get them here.
What the configs do:
Reinstate the stabilized Jool system that is usually provided by Principia (using planet packs causes Principia to ignore stabilizing the Jool system).
Moves Ovok into a high retrograde orbit around Sarnus, preventing it from causing a Principia error around year 15.