If your vehicle is showing dark brown or black fluid puddle under the front of the car, here are the most likely causes and the parts you'll need to fix each one.
The drain plug was not tightened properly during a recent oil change, or the crush washer gasket was reused instead of being replaced, allowing oil to seep out.
The oil filter was left loose, or the old filter's rubber gasket stuck to the engine block causing a 'double gasket' blowout that leaks oil rapidly.
The rubber or cork gasket sealing the top of the engine has hardened and cracked over time, allowing oil to seep down the sides of the engine block and eventually drip to the ground.
The seal between the bottom of the engine block and the oil pan has degraded from age and heat, causing a slow, steady drip of engine oil.
The seal around the front of the crankshaft has worn out, slinging oil around the front of the engine (often behind the main pulley) and dripping down.
The plastic housing or internal diaphragm of the oil pressure sensor has cracked, pushing engine oil out under high pressure while the engine is running.
The vehicle bottomed out or hit road debris, cracking the metal or plastic oil pan and causing a rapid, catastrophic loss of engine oil.