If it is indeed a freewheel, then take the wheel out and place it horizontally with the freewheel up. Dribble oil into the gap between the fixed part (that doesn't move when you spin the cogs) and the smallest cog while spinning the cogs. Keep doing this until the oil coming out the bottom of the freewheel runs clear. Wipe off all the excess oil and wrap a rag or paper towel between the inside of the freewheel and the spokes. Let that drain for a few hours so you don't have oil coming out while you ride. If it's a freehub, then you need to tell us the brand and model so somebody can tell you how to take it apart and clean/lube.