It's best to say that Venomoth should only be used in the UU metagame. It is the fastest Pokémon to use any sleep-inducing moves, not to mention that it's arguably the best sleep user in UU, together with Tangela. If you do want to use Venomoth in the OU environment, then all it should be used for is support, but you must be careful. Even though Venomoth is better than Butterfree and Beedrill in nearly every aspect (though that isn't saying much), Parasect still does what Venomoth does better.
Venomoth is best used as a lead. You will most likely put to sleep or paralyze (preferably sleep) an opponent's first counter for Venomoth. Your opponent will most likely have no choice but to then switch out to another Venomoth counter. If you miss with either Sleep Powder or Stun Spore, or you finish statusing with Venomoth, it's recommended that you switch in something to counter the current Pokémon that your opponent has out. It'd be even better if the Pokémon you sent out has a move that can induce paralysis. Starmie and Snorlax are great choices in this case since one is fast and hard hitting in the special department (with the option for direct paralysis), while the other can take blows, and can deal damage and give a chance to paralyze at the same time. If Venomoth is still active in the battle later on, use Psychic or Mega Drain accordingly.
You're not going to see Venomoth in the OU environment. You'll see it in the UU metagame where this same strategy applies. It is the fastest sleeper in UU, so you are almost guaranteed to achieve this basic strategy. Remember though that physical counterparts in the UU environment are still generally fast. Keep your guard up at all times.
Supersonic is available to turn Venomoth into a parafuser (a Pokémon that paralyzes and confuses), but it's quite inaccurate. Venomoth can't stay in for very long either. You can use Toxic, although it probably isn't worth using over Sleep Powder or Stun Spore. If you really want to be tricky and avoid situations that have Pokémon who rely on stat-up moves like Amnesia, you can use Psywave—which can deal up to 150 damage. You shouldn't use any physical attacks with Venomoth since its Attack stat isn't worthwhile.
Checks and Counters
Since the only Pokémon that can truly use a Bug-type move effectively is Jolteon, Psychic-types will murder Venomoth. The rarely seen Fire-types also eat it for lunch. Flying-types, however, are even more of a pain. You practically need to watch out for any physical hit whatsoever, as Venomoth's Defense is poor to say the least. Venomoth is on the same level as Butterfree and Mr. Mime in terms of poor defensive capabilities. As long as the Pokémon can handle a non-STAB Mega Drain or Psychic, Venomoth is going to be in some pain.