Rinse the vegetables under cool running water and dry them well. Halve the tomatoes, slice the cucumber, dice the bell pepper, and thinly slice the red onion.
In a small bowl, whisk the olive oil, red wine vinegar, lemon juice, oregano, and black pepper. Taste before adding salt because the olives and feta are salty.
Place the tomatoes, cucumber, bell pepper, onion, and Kalamata olives in a serving bowl. Pour over most of the dressing and toss gently.
Top with the feta and drizzle with the remaining dressing. Finish with a pinch of oregano and serve right away.
For make-ahead prep, store the chopped vegetables and dressing separately in the refrigerator, then combine just before serving.