There are basically two types of oil systems in vehicles, both of which sound like types of walruses or something: wet sump and dry sump.
Most cars use a wet sump system. (The more you say it, the weirder it sounds. Wet sump. Wet sump.) That means the oil pan is at the bottom of the engine, and the oil is stored there. Remember Oliver the oil molecule's lounge? It's kind of like he has a table next to the dance floor at the club. And in this strange metaphor, the dancers are pistons and bearings.
The advantage of a wet sump system is its simplicity. The oil is close to where it will be used, there aren't too many parts to engineer or repair, and it's relatively cheap to build into a car.
Some cars, especially high-performance cars, use a dry sump system. That means the sump isn't underneath the engine -- in fact, it can be located anywhere within the engine compartment. After Oliver does his job in the engine, he doesn't just drip into the lounge. He goes to the VIP room away from the dance floor.
A dry sump system gets you a couple of bonuses: First, it means the engine can sit a little lower, which gives the car a lower center of gravity and improves stability at speed. Second, it keeps extra oil from soaking the crankshaft, which can lower horsepower. And, since the sump can be located anywhere, it can also be any size and shape.
Two-stroke engines, by the way, use a completely different type of technology. Scooters, lawn mowers and other two-stroke machines have the oil mixed right in with the gasoline. When the gasoline evaporates in the combustion process, the oil is left behind to do its slick business.
Sometimes you have to do this yourself, measuring the correct amounts before filling the tank. But sometimes, as in most motor scooters, there's an injection system that takes oil from the reservoir and mixes it with the gasoline for you in just the right proportions.