I just wanted to post and say how incredibly underrated Omastar is as a late game sweeper at the moment. Many solid teams simply aren't prepared to deal with it, and I'm going to show you lot why. First of all, the first thing that comes to mind when preventing an Omastar sweep is to revenge kill it. It only has a 55 Base Speed, how hard can it be? Wrong. Omastar cheerfully outspeeds the by far most common scarfer in RU, Rotom-C, and demolishes it with a +2 Ice Beam (I've actually had opponents rage at me and call me a hacker because for some reason they think Rotom-C outspeeds :p). There are of course Scarfers that do outspeed Omastar, but they are uncommon or easily taken advantage of by its teammates, which I'll get to later.
The next thing that makes Omastar the awesome snail that it is is its bulk and typing. base 125 Defense, and resistances to Normal, Flying and Fire go a long way to actually getting up that Shell Smash in the first place. The trick is to lure in the attacks that Omastar can set up on, and at the same time have a teammate that not only does this, but also deals with the things Omastar does have trouble with. The most obvious and best way to do this is to utilize Grass-types. Alone from the synergetic point of view, Grass-types lure Entei into a Flare Blitz, Flying-types into a Brave Bird, etc. Meanwhile, Omastar can double switch to a Grass-type on moves such as Thunderbolt, Giga Drain and Earthquake. Amoonguss and Roselia are excellent choices, as the former can cripple whatever gives Omastar trouble with status (Sleep and Paralysis), while the latter can provide Spikes which can push a couple bulky Pokemon into Omastar's killing range. They also heavily dissuade Entei and other choiced Fire-types into using HP Grass. You also have to keep in mind that you CAN let your opponent kill off your Grass-type with Flare Blitz w/e, as it's just a free setup opportunity for Omastar. Roselia and Amoonguss are also excellent switchins to some of the few Scarfers that do outspeed Omastar, such as Manectric, Lilligant and Rotom-N. Hitmonlee is another case actually which Omastar has trouble with, as a Fighting Gem Mach Puch can KO on reduced Defenses, or it can flinch Omastar with the Normal Gem and proceed to knock it out with Close Comat or HJK, which is again why Amoonguss is a powerful partner, as well as Pokemon such as Golurk and Uxie.
Next I'd like to give my thoughts on the items that Omastar currently carries. Of them, I honestly find White Herb to be the inferior option. While it's nice that it nullifies the stat drops, you're not really going to have much opportunity to benefit from this. Omastar is slow enough that it'll generally Shell Smash after it takes an attack, so it won't sufer from the dropped defenses. Then, after it does Shell Smash, the recovered defenses still won't help if your opponent has a Pokemon that can outspeed and OHKO Omastar, as Electric- and Grass-type moves will KO it regardless. No, I much prefer using Life Orb or Lum Berry. Both fulfill specific purposes that you will have to choose depending on what your team struggles with the most. Life Orb obviously has less survivability than Lum Berry, but has a far greater power, as gives Omastar a 50% chance to OHKO Clefable with Hydro Pump after a Shell Smash, and get a far nastier hit on other bulky Pokemon such as Slowking (the Specs version, ie 212 HP, is OHKOd by HP Grass after Stealth Rock and with Life Orb), and is thus less reliant on teammates to wallbreak. If you DO have a powerful wallbreaker that can smash through specially defensive Pokemon (here Absol comes to mind, as it can Superpower the pink blobs, trap and kill Slowking, etc. Escavalier is also an excellent option for the same reasons, but has a far easier time switching in, not to mention DOUBLE SNAIL) then Lum Berry becomes a more appealing option. You see, Lum Berry gives Omastar more opportunities to set up on walls, such as Uxie, Clefable and more. Keep in mind that Lum Berry really only should be used in the lategame though, where everything is weakened, as Omastar just doesn't have the power to push past defensive Slowking with it. Life Orb can be used midgame due to the far higher increase in power, and that's also where most Flare Blitzes get thrown around.
I just kind of wanted to post about Omastar because it's a Pokemon i rarely see nowadays, and when I do see it it's often used wrongly, and thus doesn't live up to its potential :/