What could be wrong?

The oil light illuminates to warn you of low oil pressure rather than low oil level.

People get confused though because low oil level will sometimes result in low pressure.

A faulty oil pressure sensor could be the problem. If the sensor's OK though then the problem is likely to be more costly as intermittent low pressure could be down to a worn pump, partially blocked oil filter/pipe or general engine wear.

A garage should be able to help with a full diagnosis and repair as necessary.