Preheat the grill to 400 degrees. Salt and pepper the cod.
In a small bowl add the butter, cajun seasoning, olive oil, and garlic. Reserve half of the marinade.
Brush on each side of the cod and place on a well oiled grill.
Let it cook for 4-5 minutes. Carefully flip and cook the other side for 4-5 minutes or until it reaches an internal temperature of 130 degrees and flakes easily. Brush the reserved marinade on the fish right at the end.