Place the red onion in a small bowl and cover with cold water. Let sit at least 5 minutes, then drain (this removes some of the harsh bite).
In a small bowl or liquid measuring cup, stir together the dressing: olive oil, red wine vinegar, garlic, salt, oregano, and black pepper.
Place the romaine in a large bowl and drizzle lightly with the dressing. Toss to coat.
Add the cucumber, cherry tomatoes, bell pepper, black olives, capers, and drained red onion. Drizzle with a little more dressing then toss gently to combine.
Last, add 3/4 of the feta, and toss very gently. Add salt and pepper to taste. Sprinkle the remaining feta over the top. Divide between serving bowls and serve with additional dressing as desired.
Notes
TO STORE: Refrigerate leftover salad in an airtight storage container for up to 2 days.