A D-corps that has 5/7 bottom pair D (Kulak, Bouchard, Ceci, Desharnais, Broberg) and only two top 4 D (Ekholm, Nurse) and neither of those top 4 D are an actual no.1 is going to struggle just on the face of it.
Unless you have a systems coach that is a defensive genius (like a Trotz, Sutter, to a lesser extent Tortorella) or one of the top goalies in the game, you're going to get exposed.
That's just all there is to it. You can't expect great results from an objectively poor group of defensemen without there being some kind of other X factor at play (defensive genius coach or superstar goalie) which the Oilers don't have.