Celebi ->> Psychic doesn't have good enough coverage in OU IMO. You might want to try HP Fire or Earth Power over it to hit Scizor/Metagross/Jirachi and Heatran respectively.
Jask ->> Protect/Sub/BPass/Swords Dance @ Brightpowder is my favorite set for this. Protect first turn, then alternate between Sub and Protect until they get a miss. At the worst case scenario, you'll pass +6 Speed. You might want to pass earlier if a set-up Poke appears though, as you are set-up bait. Move those Atk EVs to Def, not that it'd matter. With SR everywhere, Sash is useless. Lefties lets you sub one more time.
Scizor ->> If you are sure that Jask will pass speed, Iron Head > Bullet Punch might be a good choice. The EVs are redundant. Maximizing your HP is always the best choice.
Starmie ->> I'd suggest a set of Recover/Spin/Surf/Ice Beam. Starmie's speed lets it check many threats, and it's rather bulky. Use this spread: Timid 216 Spd/252 HP/40 Def. 216 Speed + Timid lets you outspeed all base 100 Pokes. The rest in Defenses for more bulkiness. Lefties again, Focus Sash will NEVER BE OF ANY USE.
Dugtrio ->> With suicide leads on pretty much all teams, Focus Sash is practically useless. You might want to try and use Choice Scarf Dug. Adamant, 252 Atk/252 Spd/6 HP, EQ/SE/Night Slash/Aerial Ace.
Rotom ->> If you are afraid of Gar, you can use TrickScarf Tom, which also screws up walls. Trick/Overheat/Shadow Ball/Thunderbolt or Discharge; 252 SA/Spd, 6 HP, Modest is the best spread. Sucker Punch coming from Modest 0 Atk Rotom-C does a pitiful %42-49 to Gar anyways, and he can pretty much OHKO you back.
For a defensive Pokemon, ALWAYS MAX HP FIRST. That's the rule of thumb, which gives it more survivability on both sides. Bliss is an obvious exception.