If it's rancid oil, you need to get the oil out of the leather in order to get rid of the smell. Rubbing alcohol will dissolve the oil, and won't hurt the leather. You don't need to saturate the leather in the alcohol. Just wet a rag with it, and wipe the holster down, then repeat as necessary, giving the alcohol time to evaporate between sessions. When you've gotten the oil out, let it sit a while and see if it smells better, then give it a good cleaning with your favorite saddle soap.

__________________
S&W Forum Member #721