Anchovies--you either love them or hate them. If you fall into the former camp you'll be fighting for the little fillets on this salad. If not, leave them off and you still get a lovely oregano-flecked sliced tomato salad--the essence of summer.
Arrange tomato slices on a platter. Top with onion, anchovies, oregano, salt and pepper. Drizzle with oil and vinegar.