Heat oil and butter in an 8-inch non-stick omelet skillet on medium heat. Add onions, tomatoes and mushrooms.
Sauté until heated through. Add shrimp and cook until heated through.
Add eggs, reduce heat to medium low, and allow eggs to cook until they just begin to set.
Tilt the pan and lift up the bottom edge of the omelet so raw egg runs underneath and continues to cook.
Flip omelet, add crabmeat and cheddar cheese. Fold omelet over carefully with spatula and press until lightly golden and cheese is melted.