Cloyster's usage in SPL proves he deserves a place in OU.
Cloyster explodes, something Lapras can't do. And Explosion is a great move, even a bigger advantage than Clamp overall. I am not saying Cloy's better than Lapras though, in case anyone thought so.
Of course, Closyer is not sweeping with Clamp as Dragonite is with Wrap. First, Cloyster is never sweeping if there is an unparalyzed Starmie/Alakazam switching into you everytime, and has problems against Chansey and Lapras as well. And with the low PP, the opponent can also play around clamp cloy with swictches. Overall, I find Clamp's ability to force plays I can capitalize on to be more useful, and Clamp can help wearing down things, although Cloyster also has the potential to sweep in some scenarios.
Rest in Cloyster is ueful for one thing and just one thing: walling Snorlax, the second biggest threat in RBY. The problem comes when your opponent predicts you and capitalizes with Tauros. But if Tauros switches in in the next turn you wake up, it is a good position for Cloy since it can wake up at +75% health. Although another problem when using Rest are electric-types if you dont have Golem/Rhydon to wall them.
Zapdos is imo significantly better than Jolteon. Jolteon's higher speed may make switching out against him more difficult when your mons are weakened and can outspeed Tauros/Zam/Starmie, but overall it's imo not generally very difficult to play around it with any combination of paralysis/EQ/Exeggutor/Alakazam/Chansey and some switches.
Personally I find the thing to be nearly worthless. It's role seems to consist on taking a hit on the switch from a physical attacker and later explode on starmie.
That alone is still not that bad.