Hate to say it, but I'd like to see Omastar drop. It simply isn't very effective in this metagame, and placing it in the same ranking as Pokemon like Amoonguss and Clawitzer is too generous for it IMO.
The Shell Smash set is really mediocre at the moment. It has several good checks in Jellicent, Ferroseed, and Gurdurr and some shakier checks like Assault Vest Eelektross (this dies to a +2 Life Orb Hydro Pump, but will beat Omastar holding any other item). The rise of Jellicent in particular is troublesome for it, as it can tank a +2 LO Hidden Power Grass and retaliate with Scald. Also, offense has a ton of priority in the form of Shiftry, Hitmonlee, Druddigon, Skuntank, and other Pokemon that can easily pick off Omastar after it sets up, so those teams don't need something that can live a +2 attack. Offense just needs to be able to pressure it so it never sets up, and unless Omastar can get in on a Fletchinder or an ideal Choiced-locked move, it is going to have a difficult time setting up with that terrible defensive typing.
The hazard set is way more common than the Shell Smash set, but vs. most teams it has a very difficult time getting up more than one entry hazard. It is very easy to Volt Switch out of in the lead slot with a Rotom / Jolteon / Eelektross and finish off Omastar with a faster threat. Since lead Omastar only fits on HO, any Ground-type that you use with it (Dugtrio / RP Rhyperior) is not going to want to switch into any potential coverage move from LO Jolteon or what have you, so stopping the opponent from gaining Volt Switch momentum is difficult in practice. Fast Taunt users like Cobalion and Drapion stop it from getting hazards as well, though they both hate Scald burns and Drapion will likely lose one-on-one. Cobalion will outspeed Timid Omastar even if it activates Weak Armor, as will Durant / Virizion / anything at or above base 108 Speed. Omastar also has a type disadvantage vs. two offensive Defog users in Shiftry and Flygon, although if they try to Defog on Omastar they are getting smashed with Ice Beam. They're better off attacking Omastar after its Focus Sash is broken and using Defog later. TL;DR Weak Armor is not difficult to play around and Omastar's defensive typing leaves it susceptible to the majority of the metagame, impeding its ability to set up more than one hazard.
You can anticipate an Omastar anti-lead to gain some early momentum, but if the opponent manages to get a hazard onto the field by the time Omastar comes in, you're still only getting one hazard. I feel this Pokemon is better suited for B- or C+; it's still dangerous on paper and is a great Fletchinder punish (huge for offensive teams), but the metagame changes in ORAS have not been kind to it overall.
Forgive me Omastar D: