While I was flying on elytras Minecraft got crashed with that error

java.lang.NullPointerException: Exception while updating neighbours
java.lang.NullPointerException: null
	at li.cil.oc.common.tileentity.Capacitor.recomputeCapacity(Capacitor.scala:58)

(full log here)

After reconnecting every (not every, carpeted capacitors are still carpeted capacitors) block from different mods (vanilla blocks were not affected) got messed up: from iron chests to some shulkers, cables from simple storage network to shulkers, cables from OC to computer cases (first level), etc.

So what should I do? I have backup of world, but since then there were very many changes. I have latest version of OC (1.8.3) and Minecraft 1.12.2

To recover the world in this scenario, IIRC your best bet is to try and copy just the level.dat from the previous backup to the new one. This might help, but I can't promise anything. Make a backup of the corrupted world as well.