In a medium bowl combine orange juice, oil, lime juice, cider vinegar, oregano, salt and pepper. Add steak and toss in the marinade. Cover and refrigerate for a few hours or overnight.
Heat an outdoor grill to high heat. Remove steak (reserving the marinade) and place onto the grill. Grill steak for 3-4 minutes per side and brush with the reserved marinade occasionally while grilling. Remove from the grill and slice into thin strips.
Place a bit of guacamole onto the bottom of each flour tortilla. Top with steak strips, cheese, lettuce, and tomato.