In a large bowl, combine orange juice, olive oil, oregano, parsley, salt and pepper. Mix well. Place the tuna steaks in the bowl, turn to coat both sides with marinade; cover and refrigerate 30 minutes.
Preheat a barbecue for medium-high heat.
Remove tuna from the marinade and shake off the excess. Place tuna onto preheated barbecue and cook to desired doneness, about 5 minutes per 2.5cm for medium-rare or 10 minutes per 2.5cm for medium-well.
Alternative cooking methods
You can also cook the tuna steaks under the grill or in a frying pan.