Uber Megas? Or naw?

Mega Slowbro and Mega Salamence will both be banned to Ubers. The Calm Mind + Rest Mega Slowbro set is nearly impossible to stop unless you have Taunt or a phaser, and even then, you're opponent can just wait until they've KO'd your "counter" to bring Mega Slowbro out. Or, they can save it as their last Pokémon, in which case you better hope you have a Pokémon with Taunt still conscious. Mega Salamence on the other hand, is a very different case. It isn't impossible to kill, but Pokémon that can OHKO it cannot safely switch in. Most carry Substitute + Dragon Dance, so even if you do decide to switch, you may end up getting swept regardless. Some players even opt to invest in bulk instead of maxing its speed/offenses since they'll be boosting with Dragon Dance anyway, and the added bulk just makes it even more difficult to take down. In addition, some Mega Salamence carry Facade as their attacking move. Facade ignores the attack drop from burn and gives Mega Salamence a 140 base power (stronger than Double-Edge) flying type STAB, rendering Will-O-Wisp and Toxic useless.

Mega Salamence and Mega Slowbro are on nearly every team in ORAS OU. They're over-centralizing the metagame and will be banned.

 

I think Mega-Sharpedo is going to be pretty OP.

This would be my set.

252 Atk, 252 Spe. Jolly.

Speed Boost/Big Jaws

Item: Mega Stone thingy.

Moves:
Protect
Swords Dance
Crunch
Ice Fang/Waterfall/Thunder Fang.

 

I love Salamence but his mega is OP. I tried ORAS OU and said "well gamefreak... you gave mence a mega it didnt need!" I really hope that quickly moves mence and slowbro-mega to ubers. These mega evolutions are ridiculous.

 

Mega Salamence is straight up stupidly good, it can set up on almost anything. it sets up on Gyroball Ferrothorn, most Rotom-Ws, Landorus-T not scarf heck it can even set up on Azumarill depending on roles! I give it 2 weeks in Smogon suspect.
Mega Slowbro is Crocune to a ridiculous level, it does everything crocune does without getting critted. I can see that one gone too since after one CM and with a proper EV spread it basically can't be taken down. I mean just look at the damage calcs from common knock offers!252+ Atk Bisharp Knock Off (97.5 BP) vs. 252 HP / 252+ Def Slowbro: 158-188 (40.1 - 47.7%) -- guaranteed 3HKO after Leftovers recovery, that's Adamant Bisharp!
Mega Latias is also pretty darn good, just after some damage calcs I came up with the following EV Spread: 248 HP/68 Def/192 Spe with a Timid Nature. 248 HP Gives you an odd HP Number to take less from Sandstorm and Stealth Rocks. 192 Speed EV's with a Timid Nature allow you to hit 334 Speed, outspeeding Garchomp by 1 point(you might want to add 4 more to speed creep) and then you can honestly use Cosmic Power + Substitute + Roost + Stored Power. With that Speed you can set up Cosmic Power on pretty much anything not called Greninja(which you can set up on after 1 CP and without crits or flinches.) The rest is just dumped into Defense to get as much mixed Bulk as possible. This set is not too broken since it gets PP Stalled by Dark Types and ruined by steel type phazers and fast taunters, still pretty threatning though.
Mega Latios Does everything Latios does... doing less damage. I'd much rather stick to Life Orb Latios and not waste a Mega Slot, is good don't get me wrong but I would only use it if I'm weak to LO Stalling and to deal a bit more damage to Heatran, I guess it's a nice lure.
Mega Beedrill kinda sucks, you can't sweep with it cause priority wrecks it and you can not use it as an effective U-turner because it eats 25% + Spikes on entry, the U-turns sure hurt though but that doesn't make it too strong seeing how aggressive the format is.
Mega Gallade makes me so happy, I love that thing and now it takes on non scarf Keldeo! The only problem is how it suffers from 4 Slot Syndrom, you either run Dual STAB and not run Knock Off/Ice Punch or forget Zen Headbutt/Psycho Cut and get walled by Azumarill. I should try an all out attacking set without SD...
Mega Swampert haven't tested it, I haven't had too much trouble dealing with it on the ladder... It looks like a weaker Metagross that can have free rock polishes at the price of 1 Team Slot.
Mega Sceptile is pretty cool, Grass + Dragon + Focus Blast can be pretty destructive, specially with Entry Hazard Support, I don't find it that great right nw cause Mega Salamence being everywhere increased the amount of Ice Shards. I honestly love how it can't be paralized by T-Wave or Stun Spore, it's just hella fast and the only way to slow it down is via Body Slam. I can see it being a burden on the common Full Off Teams that rely on Thundurus-I for their Speed Control.
Mega Metagross puzzles me, I'm not sure whether or not it's too good, I mean it's incredibly good and Rock Polish can easily sweep careless people but it doesn't have too many free switch ins or free set up opportunities. Plus since M-Mence is running around, Mence + Trappers are everywhere and Sash Dugtrio gives this thing massive headaches, and so can do Magnezone if you decide on running Ice Punch over Earthquake(so a bit of a 4 Slot Syndrom there)
M-Sharpedo is just crappy, you only get one chance to pull off a sweep since you loose your speed boost. If it ever becomes a thing people will just run scarf keldeo for a while, and azumarill. Maybe it could work as an whole oppener in teams, it still packs a punch.
M-Sableye: I'm yet to test this thing too, so far I feel more threatened by regular Sableye... Though it is a cool Magic Bouncer that's for sure.
M-Lopunny looks awsome! Scrappy High Jump Kicks for everyone. It's movepool is kinda limited but I feel like just dual STAB + Fake Out + Toxic works in OU just fine. I probably wouldn't use it over Gallade because I don't like relying on High Jump Miss.
M-Diancie is a lot of fun, it does SO MUCH DAMAGE with it's stabs and Earth Power completes it pretty nicely, I feel like HP Fire for Ferrothorn isn't worth putting over Rock Polish though. It's kinda like a mixture between M-Metagross with it's Rock Polish and great moves and a bit like M-Beedrill in a sense where it gets hurt by priority(obviously not as badly as beedrill does). The biggest thing is of course being mixed.
M-Altaria is a Dragon with a Dragon inmunity and no longer weak to Stealth Rocks, looks awsome in paper but it's stats are sort of subpar, the DD Set is kinda cool but you obviously need two DDs to get some real work done. The Special Set is a lot of fun, maybe not top OU Quality but it has it's niche.
The rest of the megas are not good enough to be a real threat in OU in my opinion.
This is just me though.