If your vehicle is showing dark brown or black oil puddle under the engine, here are the most likely causes and the parts you'll need to fix each one.
The drain plug wasn't tightened properly during the last oil change, or its crush washer/gasket wasn't replaced. This allows engine oil to drip directly from the oil pan.
The oil filter may be loose, over-tightened, or have a 'double gasket' where the old rubber seal stuck to the engine block during replacement. This can cause a rapid, heavy oil leak.
The rubber or cork gasket sealing the top of the engine degrades and hardens over time. Oil seeps down the sides of the engine block, collects dirt, and eventually drips to the ground.
The seal between the bottom of the engine block and the oil pan has dried out or cracked. This causes a steady drip from the lowest point of the engine.
The main seal at the back of the crankshaft has worn out. This causes oil to leak from the bell housing area where the engine meets the transmission.
Seals at the front of the engine behind the main drive pulleys have failed. Oil often gets slung around by the belts and pulleys before dripping down.
The plastic housing on the oil pressure switch can crack, or its threads can leak. This pushes oil out under high pressure while the engine is running.
On turbocharged vehicles, the pressurized oil feed or return lines supplying the turbo can degrade, crack, or have loose fittings, causing a severe leak.