Music
I think there was a piano piece that was originally from Oni that made it to the score of HALO. It was Siege of Mathawhatever or something like that. It's very good.
shareIt's called "The Siege of Madrigal", and it's not from Oni. It's from an even older Bungie game: Myth.
I agree with you, though, it's a pretty cool tune. I remember flying up above the control room with my Banshee flier for the first time, walking around on that middle pylon, and when I heard that music I was like... "Wow, I just found an easter egg! Sweet!"
Yes, it's actually an easter egg. You have to walk onto a specific part of the level "Assault on the Control Room"... in Halo, of course.