the oil light on 2005 Mercury Mariner

i changed the oil but the light still comes on (theres knocking in the engine)

Asked by for the 2005 Mercury Mariner
Sounds from your question that the engine bearings are faulty and you'll need to fix the bearings or replace the engine. The choice depends on condition of the rest of the engine. You need to find a Mercury/Ford specialist for a diagnosis and estimate. Usually, by the time the bearings are worn out, the rest of the engine's worn out too but sometimes you can save an engine with bearings and a crankshaft if the crank's damaged, and it probably is by this time.
If you want to find a specialist, here's a directory link for you: