In a blender, combine lime zest and juice, orange zest and juice, oil, garlic, oregano, cumin, salt and pepper; blend until smooth.
Reserve 3 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, about 10 minutes (or cover 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 broiler), turning once, until desired degree of doneness is reached, 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.
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