So, I haven't had much of a chance to mess around on the ORAS OU ladder, but I've been trying out Mega-Aerodactyl as an anti-meta pokemon to handle the new Megas. It has been working fairly well - it outspeeds all of them, and its STABs net easy OHKOs on Beedrill, Lopunny, Gallade, Pidgeot (yay No Guard Stone Edge!), Sceptile and Glalie. It also has a pretty good chance of OHKOing Sharpedo if rocks are up, and deals solid damage to Salamence (though not as much as I'd like - with Intimidate, Jolly Mega-Aero deals ~60% damage, so it misses the OHKO even after rocks).
Mega-Slowbro hasn't seemed like as much of a problem as I'd expected, although I've only actually faced it once in the handful of battles I've played - most of that impression is based off of watching battles. It's still scary, but not overwhelming.
Most of the Mega-Swampert teams I've seen have been trying out full rain teams. That's alright, I suppose, but it's pretty easy to dismantle. I think a smaller rain core on top of a bulky offensive team will actually be scarier for Pert.
Megagross, on the other hand, has really surpassed my expectations. It does good work in every single battle I've seen it used. That bulk is really what carries it.
Pidgeot still seems pretty garbage in OU, but maybe I just haven't seen anyone use it well.
LO Sharpedo is stronger than Mega Sharpedo and the latter has no Type Change or much useful Ability; no point of wasting a Mega Slot.
LO Sharp isn't actually stronger than Mega-Sharp when you take Strong Jaw into account:
252+ Atk Life Orb Sharpedo Crunch vs. 252 HP / 0 Def Arceus: 152-179 (34.2 - 40.3%) -- guaranteed 3HKO after Stealth Rock
252+ Atk Strong Jaw Sharpedo Crunch vs. 252 HP / 0 Def Arceus: 195-229 (43.9 - 51.5%) -- 8.6% chance to 2HKO
I still don't think it's worth a mega slot, especially since it's not a big enough damage boost to sweep on its own, and it's still fragile enough to be forced out by priority or bulky attackers (after which point it loses Speed Boost). But it's definitely stronger than standard Sharp, especially now that it can comfortably run Ice Fang over Ice Beam. Waterfall is obviously weaker, but Crunch is a far more spammable move anyhow.