If you want to use an offensive Ghost type in OU, Dragapult is better, and as an offensive Grass type Kartana is better.
U-turn is better on Decideuye if it is, for some reason, still going to be used. It's much better to just predict the switch to Mandibuzz or Heatran and then switch to Tapu Koko or Garchomp than it is to use Brave Bird (which can easily be Roosted off by Mandibuzz and is resisted by Heatran).
As for items, it depends on the set. U-turn is better on Choiced sets, while something like Life Orb or Spell Tag can be used on Swords Dance sets and you can drop U-turn or Brave Bird entirely.
Decidueye @ Choice Specs
EVs: 252 SpA / 4 SpD / 252 Spe
- Leaf Storm
- Shadow Ball
- Giga Drain
Decidueye @ Life Orb / Spell Tag
Ability: Long Reach
EVs: 252 Atk / 4 Def / 252 Spe
- Leaf Blade
- Shadow Sneak
- Swords Dance
(please use Decidueye in NU and in NU only)