OHKOs don't bring HP to 0, they actually do damage. They do damage equal to the target's max HP (which matters for purposes of passing Substitute). I think Endeavor also does something like damage = their HP - your HP.
I hear about these multi-billion damage reports, but I don't know how that works out. I can't get anything above 142,597,368. This is what I did:
(((((((100 × 2 ÷ 5) + 2) × 1440 × 4912 ÷ 50) ÷ 1) × 1) + 2) × 2 × 2 × 100 ÷ 100) × 1.5 × 2 × 2 × 1
Level 100, 1440 power (Defense Curl, Helping Hand, and 5th turn Rollout), 4912 Attack (614 Attack Shuckle from Power Trick, Pure Power, +6 Attack), nothing in mod 1, CH, Metronome to give x2, max damage in the random number, STAB, 4x weakness, nothing in mod 1. Am I leaving out a way to increase this damage, or did I calculate anything wrong?