When your oil light is coming on when your engine is idling the cause is usually either a bad oil pressure sensor or low oil pressure.
Sometimes the oil pressure sensor can fail and cause the computer to think the engine has low oil pressure which then triggers the engine oil light.
When your engine is idling the oil pressure is lower than it would be when the engine is at a higher RPM.
The low oil pressure could be caused by low engine oil level, bad oil pump, clogged oil filter or other internal engine damage.
If your oil level light is coming on you should check the oil level and make sure it's full and if the problem persists change the oil filter if you haven't already.
If that doesn't fix it then you may have a bad oil pump that needs replaced which is a time consuming job because you have to remove the oil pan to get the oil pump off the engine and replace it.
If the oil level light continues to come on I would recommend taking it to an auto shop so they can figure out exactly what is causing the problem and fix it before it leads to major engine damage.
However sometimes it could be as simple as the oil pressure sensor needing changed which is what I change on some vehicles experiencing this problem.