I have a 2001 BMW 320i , 92,000 miles , 2.2 L. recently I had the oil sensor replaced along with an oil service. Since then I have been toping up the oil every 4-5 hundred miles. The oil light kepps coming on but there is no sign of any oil leak. What else might be the issue.

That oil has to be going somewhere. If it is not leaking out, it is being used by the motor somehow. return to the previous mechanic and FIND IT!!!!!!!