Slice the tuna fish as thinly as possible. Firming it up in the freezer helps. Brush each slice with the oil, and season lightly with salt and pepper. Leave for up to 3 hours, covered, in the refrigerator. Just before serving, brush with the orange juice. Decorate with the herbs and serve. You can serve this as finger food by placing the tuna fish on endive or Little Gem lettuce leaves.