The primary explosive was supposed to be ANFO (Ammonium Nitrate Fuel Oil) which is created by mixing the proper amount of diesiel fuel with the fertilizer prills. McVeigh is also reported to have bought a barrel of nitro-methane, a very expensive, exotic racing fuel that can act as a significant sensitizer/enhancer for explosives. Mixed with ANFO, it should make the explosive much more effective.
All of this information is easily available to anyone who studies the industrial uses of explosives.
As I know so little about explosive mixes,I had only my memory.It probably stuck because it was a Texas source and Oklahoma felt awfully personal to me.