Oil is lighter than water.  ...

Oil is lighter than water.  If the water can flow to an area of smooth flow rather than turbulent, then the oil is on the surface and the water taken from the underside of a barrier, or a sloped beach with a scraper could be used to separate it.  However some will become emulsified.  The oil will end up harbouring bacteria, which will release back into the water.  Another could be the use of a DAF plant to separate the oil.  However you will not get rid of 100%, oil and as I said, it will end up causing increased bacterial growth.