Playing for the first time. Was clearing out the bandit camp near Fort Locke or whatever, and after finishing the battle with the Bandit Sergeant, his wizard, and a couple of his thugs, my half-orc barbarian and team were pretty low on health. Suddenly I see a red -40 and my half-orc collapses, and just as suddenly jumps back on his feet fully healed (I think there was a sound of magic something or other at the time).

What happened here?

Ah yes, perhaps it was rage. Would the added CON have accounted for 40HP? So all dead characters are revived after battle in OC, even playing by "Hardcore DnD" rules? Or is it only the main character that's special?

No, not anywhere close to level 10.