Originally Posted by boots
When faced with a new bridle or halter that is stiff, I put the whole thing in a container of the oil and leave overnight.
I've heard lots of people do the same thing, so I ought to point out over-oiling like this will weaken the leather. Basically the fibres swell to the point they break apart, which is why the item appears softer or more flexible. Problem is it won't last so well or be as strong - oiling really needs to be done sparingly.
As to the original question it's traditionally oil before soap. Soap, applied on a dry rag so it doesn't lather and block buckle-tag holes, helps seal oil inside the leather for longer.
So it's oil first, surface treatment after for me