Cloyster can hold his own against Snorlax. He has increased in popularity as a Tauros counter considering his massive Defense, though a lack of HP hurts him to an extent. Clamp, as all Wrap-like techniques, is a great asset, and Explosion is always good. Too bad all Cloyster usually does in OU is blow up on Starmie. Don't let this hold you off—Cloyster is great to toy with.
Clamp works like Wrap, except it sacrifices some accuracy (it is 75% accurate) for increased power and special STAB (Gengar doesn't screw it over like it does for Wrap). It cleans up paralyzed Pokemon really nicely, and slower ones such as Chansey, Exeggutor, Snorlax, and Hypno. Additionally, if someone switches into Clamp and the attack connects, Cloyster can switch out next turn while the opposing Pokemon is unable to attack. It's definitely a great late-game (or even mid-game) lockout move.
Blizzard is Cloyster's highest powered STAB special attack. Anything that Surf is effective against, so is Blizzard (in the OU metagame anyway). Furthermore, you only suffer a roughly 10% accuracy depletion in favor of an extra 25 Base Power. This is one of two moves Cloyster will use to finish off his opponent after Clamping them to within KO range. He won't get enough attacks off to worry about PP, so Ice Beam isn't necessary.
Hyper Beam is his other move, as it takes care of some threats that Blizzard can't. For example, after Clamping a paralyzed Chansey, Alakazam, or Starmie, Hyper Beam would be the best choice to finish them off.
Explosion is the best choice for the last moveslot. After Clamping a few Pokemon into watery graves, Cloyster can take down yet another with this self-sacrificing move. It's also good as a last resort, or as surprise tactic to get rid of something that could become a huge hurdle for your team (such as a Starmie or Tauros). It could be a good strategy to blow up Cloyster when you are 2-1 at the end, as well.
Double-Edge is an option if you fear opponents being able to switch Gengar or a Rock-type into an Explosion (or Hyper Beam, in the case of a Rock-type). Rest could work as well to keep Cloyster around even longer, making him more of a physical tank. Surf is another possibility if you want extra special damage (beware that you will have a hard time dealing with Chansey and friends). Supersonic, Mimic, Substitute, and Reflect are also possible fun ideas to throw on Cloyster should he have time to mess around.
Checks and Counters
Starmie and Lapras are Water-types that resist Cloyster's Ice-type attacks and can fire back with super effective Thunderbolts. Slowbro is another Water-type, and much more defensively oriented. He shakes off Cloyster's attacks very easily and can paralyze or wear him down with Psychic. Victreebel and Venusaur can put a dent in him with critical hit Razor Leafs, but beware of the STAB Ice Beams/Blizzards. Jolteon can fry Cloyster with its Thunderbolts, as can all the other Electric-type Pokemon; however, be careful switching in Zapdos, as its Flying typing gives it an Ice-type weakness.