Here's why:

* Liquids are incompressible. This means they resist changes in volume under pressure.

* Oil is a liquid.

Therefore, a specific amount of oil will occupy a specific volume, even if the pressure changes. This is why we can measure oil in liters, gallons, barrels, etc.

Important Note: While oil has a definite volume, it is slightly compressible, meaning its volume will change very slightly under extreme pressure. However, this change is negligible for everyday purposes.