Preheat grill pan or oven broiler (with the rack 3 inches from heat source) on high temperature.
Combine oil, garlic, lemon juice, and oregano, and brush over tuna steaks. Marinate for 5–10 minutes.
Meanwhile, combine all salad ingredients. (Salad can be made up to 2 hours in advance and refrigerated.)
Grill or broil tuna on high heat for 3–4 minutes on each side until the flesh is opaque and separates easily with a fork (to a minimum internal temperature of 145 °F).