Hey guess what!? I play CoD more than Halo, but I'm not an annoying fanboy. I love CoD more, but I like how involved the Halo Community is...
My theory for the ONI building blowing up is
During Dutch's Flashback, maybe the covenant are surrounding the building With Phantoms and Banshee's and stuff. So its Dutch and Mickey to blow it up. All the info in the Building is Transferred to another location. So they must plant explosives in some locations like Archive rooms, Libraries, etc. So then after they have planted the goods, they escape by Pelican on a near by landing pad, which Mickey pilots away from the building to safety.
This is just my short theory, but I'd like to hear what you guys come up with....
[Edited on 07.30.2009 6:44 AM PDT]