More likely, the oto died for other reasons, and the platies ate the corpse. I doubt they had anything to do with it's death. All fish will eat dead ones--even herbivores.
Otos are very sensitive fish. They travel poorly, and seldom arrive in great shape at the LFS, much less out tanks. If you want another (and they do better in groups--try at least 3, if possible), put down the money and have the store hold them a week. The survivors after a week should be healthy enough to travel to your house, and go into a cycled, healthy tank without problems.
