You'll typically find it in one of these locations:
* On top of the tank: This is common for larger, tank-style compressors.
* On the side of the motor: Often near the motor's crankcase. This is more common in smaller, direct-drive compressors.
* Underneath a cover: Some compressors have a removable cover that needs to be opened to access the oil fill port.
Always consult your air compressor's owner's manual. It will provide a diagram showing the exact location and instructions on the type and quantity of oil to use. Using the wrong oil can severely damage your compressor.