You may not realise that you have an oil leak until you take your car in to a garage to have something else done or notice oil on your driveway where you usually park. Oil can come from a number of different places on the engine. It may be coming from the gear box, the rocker cover or the oil tank itself. There are a number of different pipes and components that the oil runs through, so it any of these are damaged, it will cause the oil to leak out. If the leak is only small it may take some time before you have to top up your oil levels. If the oil leak is coming from the gear box you may not realise until it is too late and the gear box is already damaged.
If you notice oil coming from under the car or if you notice it sprayed on other components under the bonnet then you need to take it in to a garage as soon as possible for them to diagnose and fix it. If left, it may very quickly lead to further damage which can then be very costly. Often the oil is changed when you have your car serviced and oil leaks are usually picked up at this point, but it is worth always checking where you usually park the car for any new drips of oil.