Owls do in fact sometimes eat snakes, especially the screech owl which tends to grab smaller sized snakes and bring them back to the nest while they are still alive. These snakes serve a very important purpose in the nest of an owl, eating all of the vermin which accumulate as a result of the decaying animals which serve as prey for the owls. This keeps the nest nice and clean.
A lot of smaller animals such as raccoons, snakes, squirrels, and even some insects are fair game for the owl. While the diet of an owl depends on the individual species, snakes are definitely a common delicacy for these birds of prey.