For starters, OK City was a political act.
The others are all murder suicides.
Pretty big difference, actually.
Secondly, there are a number of less publicized mass shootings. If you add them all up, they exceed OK City.
edit: I do waver on banning or not banning. Like, given resistance to background checks (several local sheriffs have refused), it might be easier to simply ban weapons across the board. OTOH, I do understand why law abiding citizens might enjoy the rifle, so I'd like to create a better regulatory system that kept the rifles in law abiding hands and out of the hands of random psychos. However, several local government agencies refuse to participate, so that undermines it.