In a blender, combine lime zest and juice, orange zest and juice, oil, garlic, oregano, cumin, salt, and pepper; blend until smooth.
Reserve 2 Tbsp marinade for serving. Place steak in a wide, shallow bowl; pour remaining marinade over steak and let sit, uncovered, at room temperature while you heat grill (or cover bowl and refrigerate for several hours or overnight).
Heat grill (or broiler) to high.
Remove steak from marinade, brushing off any solid bits. Grill over direct heat (or under the broiler), turning once, until the desired degree of doneness, about 5 minutes per side for medium-rare.
Thinly slice steak across grain and drizzle with reserved marinade; serve with mango slices and lime wedges.
Serving Size: 3 oz steak, 1/3 of a mango and 1 tsp marinade drizzle!