Oliver was holding in his hand a Calico M950 9mm pistol with a 50 round magazine. One bullet, if placed correctly, can kill a person. He had used 4 bullets thus far since the magazine was last filled.

There were 27 people on the bus, all of them with anywhere between 3 and 45 feet of distance from him. A single bullet travels at 1290 feet per second.

Oliver has a small reaction time and a trained aim. He could hit 4 out of 6 3-inch diameter targets at a distance of 100 feet. He had a powerful pistol with 46 bullets left. That minus 27 is 19. Oliver would have 19 extra bullets assuming he hit all of his targets, or he would have 19 bullets to quickly correct any potential errors.

Considering all odds, Oliver would have at least 10 rounds or more left in his magazine if all goes correctly.

He would have plenty of time to kill them all, it would take five seconds for an officer to make it to the bus, 15 if they all made a mad rush from varied distances like they probably would.

The odds were in his favor.