Celebi is in OU because of:
- Great balanced overall stats
- Diverse move pool
Celebi has a base 100 in every stat making it a very useful Pokemon.
Also, it has moves like Heal Bell, which make it an effective Cleric, given the high number of sets in OU based on status affliction.
Smogon's page on Celebi lists that it can be effective in straight up offensive, defensive as well as Baton Passing teams, thus making it OU.
It is bulky enough to take a few hits, strong enough to hit back, fast enough, well, you get the idea.
A common moveset that one generally sees on Celebi on PS! is:
- Leaf Storm
- Heal Bell
- Recover/ Toxic
So you see, Celebi makes good use of its stat distribution, so that is what keeps it in OU.