Thaw tuna, if frozen. In a small bowl combine olive oil and orange juice. Brush tuna with oil mixture. For a charcoal grill, grill tuna directly on the greased rack of an uncovered grill over medium heat for 4 to 6 minutes or until tuna is slightly pink in the center, turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place tuna on the greased rack over heat. Cover and grill as above.) Remove tuna from grill. Cover and chill tuna for 1 hour or until completely chilled. Slice to serve.
Meanwhile, in a small bowl, combine mayonnaise, onion, chopped orange, rosemary, and pepper. Divide tuna portions among 8 serving plates. Top each tuna piece with 1 tablespoon of the mayonnaise mixture. In a small bowl combine orange peel and snipped rosemary. Sprinkle atop each serving and serve with crackers if desired. Makes 8 servings.