OHKO moves, the majority of the time are not reliable. The formula for calculating its accuracy is:
Accuracy=[(level of user)-(level of target)] + 30
Therefore, your ohko move won't have perfect accuracy until you seventy levels ahead of you opponent, and won't even hit half the time until your 20 levels ahead, at which point, its just easier to ok your opponents with regular attacks.
However, there is ONE exception
Articuno, the legendary bird of winter, is an interesting Pokemon. While it has decent stats all around and good defenses, it's attack and special attack are nothing notable. Also, its movepool is pretty much limited to ice and flying type moves. However, it does get mind reader, a move that guarantees the next hit, and sheer cold. Use mind reader first, then your guaranteed to ko your opponent with sheer cold next turn. However, this is gimmicky and requires set up, and articuno's weaknesses to multiple common types often prevents this from working. Smeargle, who can learn any move, can also do this, but its rather poor stats limit its viability.
So in short, ohko moves, almost all the time, aren't good.