I know that they would have had access to at least olive oil, but wouldn't acquiring enough olive oil to supply several hundred soldiers be rather expensive? Or did they simply use unhardened leather?
Well. oil would be cheaper than leather but in what process would you think that oil hardens leather? Oil soaked into leather is a lubricant and softens it and makes it more flexible.
If you mean to manufacture leather armour accessories, the leather is boiled in water causing it to shrink, thicken and become plastic and mouldable until it cools and dries hard.