My wife has a 2004 Mazda 6, 2.3 liter 4 cylinder. When the car is parked, oil doesn't pool underneath. After driving for about an hour, we need to add about 1-3 quarts. With this issue, where could the oil possible be leaking from?
Oil has a couple of places it can go.
It could be a leak that only happens under pressure (IE the engine running) but for that amount the underside of the car would be covered in oil and should be very obvious by looking under the car with a flashlight.
If it's going out the tail pipe you would see obvious blue smoke whenever the car is running.
If it's going into the radiator a simple check of the coolant level, color, and quality with let you know.
If you have a plastic valve cover, it can become slightly warped over time and oil can end up being forced out under pressure. This is unlikely, as @Larry said it should be rather obvious looking under or into the engine compartment.