Sprinkle the fish with 1 teaspoon oregano and salt. Marinate in refrigerator 2 hours, turning occasionally.
Cut 2 slits into each steak. Insert 2 garlic quarters into each slit. Place the fish on a grill rack coated with cooking spray. Grill 4 minutes on each side until fish is medium-rare or until desired degree of doneness.
Combine 1 teaspoon oregano and the juice in a medium bowl. Dip grilled fish into juice mixture; place on plate. Add oil to remaining juice mixture; stir well with a whisk. Spoon juice mixture over fish; sprinkle with pepper. Garnish with oregano sprigs, if desired.
Wine Note: There's a lot of lemon in this dish, so be sure to choose a white wine that has a bit of citrus zing itself. (Avoid red wines; they clash badly with lemon.) Many fresh, light whites from around the Mediterranean would be ideal. Try, for example, Bodegas Morgaío Albariño, one of Spain's most exciting whites (about $19).