I haven't looked into it yet but the Raid Boss Counter list says Kyogre has a ~105% TDO in neutral weather. Onix is double weak to Water, while also being double weak to grass. I see it being beatable, its defense is certainly higher than Claydol's but it also has much lower attack, far less threatening moves, and a double weakness.
Soloing the new Tier 3 raid bosses
How difficult are they and what attackers do we need?
Alakazam is familiar (and as tough as usual).
The day before Omastar was removed, I caught a wonder Treecko and hatched a perfect Bulbasaur to complete my team for it.
Guess Kabuto would be slightly harder than Omastar (despite both being doubly weak to grass) due to more defensively oriented stats. [EDIT: Got their stats mixed up; Kabutops is more offensively oriented and thus easier.] Onix may be impossible to solo with the same stamina as other tier 3 raid bosses but ridiculous defence.
As for Aerodactyl, I have a TS/WC Raikou, a two WF/HP Gyaradoses, a WG/HP Vaporeon and a RT/SE Golem (all level 30 wonders); wondering what to put last.
Answers
Have you got yourself a Aggron or Metagross to round out that Aerodactyl team? Both would make great tanks, especially with steel movesets.
Onix should be cake, as WaterYouWading4 mentioned, due to his double weaknesses to some of the highest DPS mons in the game (Kyogre, Gyarados, Vaporeon, and Venusaur with that absurd charge move). Certainly easier than Claydol.
Omastar and Kabutops will require you to keep an eye on breakpoints, though with FP Venusaurs in your squad they should both be very doable.
Kabutops has less defense than Omastar and shares the same double weakness. It's easier, your Omastar team will destroy it.
Onix is doable with level 30 Kyogres or level 35 Kyogre/Gyarados/Venusaur/Exeggutor. Fairly tough.
For Aerodactyl I'm again recommending you to put the DT/OR Dragonite on the last spot. Your team is good enough that you will often not even need the sixth member. Would recommend dodging AP with Gyaradoses and HB with everything.