It depends what you mean OP. Some games like Zelda: LttP or Tomb Raider can be completed in less than15 hours, but, I can tell you, when I was playing these games as a kid, they probably took me 500 hours to beat, because I was constantly getting stuck and exploring every last inch of every area. I'll go with some that are very easily completed in under 15 hours:
Metal Gear Solid
Super Mario Bros. 1, 2, 3
Sonic 1, 2, 3&K, CD
Nights into Dreams (not 100%)
Burning Rangers (not 100%)
Enemy Zero (if I remember correctly)
Ghouls n' Ghosts
Super Ghouls n' Ghosts
There are too many to list