Since it was baked on a screen, I'm guessing that the oil might have been from the dough ball itself. Pressed dough balls are literally dripping in oil, and I've seen even hand tossed dough balls dripping in oil.I would seriously doubt that anyone would go to the trouble, at the pizzeria level to apply oil to the bottom of the crust. To the inside bottom of the crust yes, since when used this way it helps to reduce moisture migration from the toppings into the crust (especially helpful during DELCO conditions). Some pizzerias do brush the outer edge/rim of the baked pizza with olive oil to improve the appearance and flavor of the crust (especially the edge).
Did you happen to get a look at the screens the pizzas were baked on? If they were extremely dark/black in color and appeared to be carboned up, there is a possibility that they were oiling the screens to compensate for gunked up screens, which do not exhibit the best release properties, some of that oil would be transferred to the bottom of the pizza skin as it is placed onto the screen.
Tom Lehmann/The Dough Doctor