Cut tomatoes into wedges over a large serving bowl so that any extra juices land in the bowl with tomato wedges. Add onion slices and oregano, then season liberally with salt; mix to combine. Allow to sit at room temperature until juices have released, at least 20 to 30 minutes or up to 2 hours.

Mix in bell pepper, cucumber, and olives just before serving. Top with slice of feta cheese and drizzle with olive oil.