1. Call him Octopus for no reason at all...
2. Paint Decepticon insignia's all over the Autobot base and its individuals.
3. Make lots of remarks on how his new TFP body makes him look chubby... Constantly. Make up annoying nicknames like "Beefcakimus Prime" or "Chubbimus."
4. Rig the base doors with a set of giant speakers to play "The Touch" every time he walks in.
5. Lead Optimus into a room and lock the door behind him, then through another door in the same room, let all the fangirls in, then lock that door.
6. Somehow find a way to hack into the security system, look for anything embarrassing you can find on Prime and play it over every system in the Ark...and on Cybertron...and on the Nemesis...
7. Introduce him to Fanfiction.
8. Ask him if he is Luke Sky Walker from Star wars with that glowing stick of his...
9. Paint a huge picture of Megatron on his chest while he's sleeping and blame it all on Ratchet.
10. Hug him.
11. Tightly.
I'm sorry, I would have done more...but Prime is a really hard 'bot to annoy :P Anyways...Who do you want next? Bulkhead or Arcee? ;D Bu-bye!