Why? >_> If you need us to name some good initiators for you then it doesn't matter if you know them. The best thing for you to do is to just try initiating several different ways with several different champs and see if it'll actually work alright with Orianna.
A Thresh who's keeping up on his soul count can pull off some mean initiates. (Well, he can also initiate if he's behind, but then he's gonna die instantly.) Urgot works if he hasn't built glass cannon.