Holden 2003 ss 5.7 v8

When I put my gear on D (Drive) a check oil comes on? and it goes off when I put my Gear on N (Natural) whats that?? my car is Holden 2003 SS 5.7 v8, It has an Engine missfiring due to a faulty ECM but I got it fixed. Now I have Check oil comes on when I put my gear on D. Can you help me please? I really want to know whats that might be before I go and see a mechanic because I don’t want to get fooled. Thanks !!!