If you are using a non-OEM (Fram, etc) oil filter the oil light will come on sometimes. Fram doesn't use the right valve inside the filter and it can cause you to have low oil pressure.

You need an OEM oil filter, Bosch, Mann, Mahle. Anything else (other than maybe Wix or Napa Gold) won't have the right valve. Autozone carries the Bosch filters for about $5.