Like bartamos said, oil canning is when the metal is somewhat stable in two positions, popped in or out. When you push on it and it "pops" and goes to the second stable position, and then pops back when you release the pressure that's a true oil can. It can also stay in either position and need a nudge from the opposite side to go back to the first position. If it doesn't pop it's not an oil can, but just a soft spot.
