O - Oil
Oil - Every car must have reservoirs of oil to lubricate and cool the moving parts of its engine and transmission. It's vital to regularly check that the oil is at a sufficient level. If the level drops too low, you could cause expensive damage.
Also, note that not all oils are the same. When changing or topping up your oil, always check your car's manual to see which is recommended for your model. High-performance cars, for example, may need more expensive lubricants than a standard family car.