Heat a small amount of the olive oil in a non-stick skillet over medium heat. Season mahi mahi with oregano, salt, and white pepper; cook in skillet until the flesh flakes easily with a fork, 7 to 10 minutes per side.
While fish cooks, whisk remaining olive oil, milk, and creamy pesto sauce mix together in a saucepan over medium-low heat; cook, stirring regularly, until it begins to thicken, 3 to 5 minutes. Remove saucepan from heat and stir crabmeat into the sauce.
Transfer cooked mahi mahi to plates and spoon sauce over the fish.