all steel is not oil hard. You have air and water hard steel as well.
The difference would be in the time it takes it to cool not what is in the oil.
Air hard steel uses a longer cooling time where water hard steel uses a quicker cooling time. Oil hard is between the two as far as how fast you need to cool the steel.
Air hard would have a timer on the oven after the heat was turned off and let cool inside the oven.