DTilt, OoS or to punish an air dodge are your best bets. Be aware that whiffing it against a good player will usually get you killed, and they'll be doing their best to make that happen. I find that the strongest advantage you get from a KO Punch is the threat of it rather than the OHKO itself. Think about how your opponent is trying to bait or hit you, what he's doing to prevent getting hit at all costs, and what you can do to abuse it. Abuse the fact that they're playing defensively but too afraid to shield to rack up damage. If you can get them into the air, they'll be afraid of getting KO Punched on landing - abuse that too. If that eventually gives you an opening, that's great, but the first thing on your mind should be to take advantage of the threat of a KO punch even before finding a way to actually land it.