Is {Bitterblossom} very good in EDH? I wanna run it in my {Oloro, Ageless Ascetic} EDH but I'm not sure. Thoughts?
Depends on the deck. It might not be good for your deck
By itself it's probably not worth othe current ridiculous price tag. With multiple opponents at 40 life each, and you with only a single player's worth of 40 life, you're going to kill yourself before your faeries can make a big enough impact to win the game.
Now, you mention Oloro, which negates the suicidal part of the problem, but you're still left with "oh, my 3 friends have a total of 120 life I need to take out, and my butt-load of Tinkerbells is pissing them off and making me a target." And this is, of course, ignoring the progress-hindering {Wrath of God}s slowing you down that EDH loves so very much.
But, the card has potential to synergize so well with certain deck types. If your Oloro deck contains a "Soul Sisters" life gain motif, Bitterblossum is an extremely reliable token engine that triggers all of those {Soul Warden} type triggers! It would work well with {Blood Artist} as well, or really any card that does stuff when another creature enters or leaves the field. A flight-based deck with {Favorable Winds} and {Gravitational Shift} type stuff could push them into overdrive, too. Lots of options in EDH!