During my bomb classes that I teach for ESI and other groups, the question of the Oklahoma City bombing always seem to come up. Here is a version of an Engineering animation of the blast, notice the blast tears into the structure for a few floors in the front, then as the columns fail along with the 4 foot by 4 foot steel reinforced beam running on top of the exposed columns is broken up, leaving nothing holding up the rest of the front of the building, and the failures resulting in gravity produced the rest of the damage to the building. In the second set of columns in from the blast only one gave way, the other are further into the building that you see failure was the elevator shafts to the building. When people talk about progressive collapse of a building, they are pointing to failure of the second row, or even third or fourth rows of columns, which in this case did not fail. You can see the gravity pulling down parts of the building , but if you go back and look, the second row of columns held, to the point peoples desks could be see still in place just the other side of the columns that did not fail. If you watch some of the first videos of the scene, you can see people walking up and looking out from the parts of the building past the second columns. I have seen a few versions of the engineering animations, but this is one of the most accurate available.
from CTI Consulting https://www.youtube.com/watch?v=EphsWl6ZbEQ