Whisk the orange juice, lime juice, garlic, cilantro, jalapeno, cumin, oregano, oil, salt and pepper together in a dish or a bag.
Add the flank steak, turn to coat, and refrigerate for 2 to 4 hours. Do not go past about 8 hours, because the citrus acid will begin to break the surface of the meat down to a mushy texture.
Take the steak out of the marinade and pat both faces completely dry with paper towel. Let it sit at room temperature for about 20 minutes while the grill heats.
Heat a grill or a heavy cast iron pan until it is genuinely hot, hot enough that a drop of water skitters and vanishes rather than sitting and steaming.
Grill the steak undisturbed for 4 to 5 minutes a side for medium rare, until a dark crust has formed and the centre reaches 130F to 135F on an instant read thermometer. Flank is thin, so this happens fast.
Move the steak to a board and rest it for a full 10 minutes before cutting.
Find the direction of the long muscle fibres, then slice thinly straight across them, at a slight angle. Serve with lime wedges.