Oracle Text

At the beginning of each upkeep, if you control no Snakes, create a 1/1 black Snake creature token with deathtouch.

Card Rulings

10/17/2013 Ophiomancer’s ability checks if you control a Snake at the beginning of each upkeep. If you do, the ability won’t trigger.
10/17/2013 If the ability does trigger, but you control a Snake when it tries to resolve, the ability will do nothing. No Snake token will be created.
10/17/2013 Ophiomancer’s ability considers any creature you control with the creature type Snake, not just the tokens Ophiomancer creates.