The picture said they were washing body parts off the bus, so I assume the buses couldn't have been too far from the explosion.
I just saw an OU Daily article that said there were also buses on the south oval, which would be near the explosion. The picture they had appeared to have been taken in daylight (today, I assume.) This would put the bomb between the buses in the pic and the stadium. But it is not clear to me from the reporting exactly where the bomb went off. I think still pretty far from the buses (for window breakage.)