Pretty sure the OOS there is coming from two different RNG results on two different machines. Generally speaking OOS are closely related to RNG-related events. That's why turning off random events helps, since every time a random event occurs there's another possibility for another RNG results mismatch.
Other things that use the RNG, like automating exploring and great person births don't cause OOS (to my knowledge) because other players don't need to roll the RNG for those - it only affects the one player.
By the way, I have no idea how to fix the problem. I'm only repeating here what I've heard and what makes sense to me.

You can't turn off combat

but I suppose if you played an Always Peace game with No Random Events it might just be possible to be 100% sure of avoiding OOS.
P.S.
This thread might be of interest for further research.