Without turning this into a blasting seminar, it is my understanding that simply pouring oil on fertilizer is possible, but that ANFO takes a hefty punch to get going in the first place, and that simply pouring fuel oil into it is not the path to blasting success.
The OKC blast was remarkably strong. Shoddy ANFO would only have spread fertilizer around in a big mess.
IIRC, ANFO is only 40% of the punch from an equal amount of TNT.