Crumble enough feta to measure 1 cup and spread evenly in a shallow 2-cup flameproof baking dish (roughly 9- by 6- by 1-inch) or a small deep flameproof platter.
Thinly slice tomatoes crosswise and arrange, overlapping if necessary, on feta.
Drizzle oil over tomatoes and sprinkle with oregano and pepper.
Broil mixture 2 to 3 inches from heat until cheese is bubbling, 5 to 8 minutes.