Cut the cucumbers and tomatoes into bite-size pieces, about 1.5 cm / 1/2 inch cubes. Place them in a 4-quart / 3.8 L mixing bowl.
Add the sliced red onion and olives to the bowl with the cucumber and tomato.
In a small bowl, whisk together the olive oil, lemon juice, red wine vinegar, oregano, salt, and pepper until combined.
Pour the dressing over the vegetables and toss gently to coat everything evenly.
Let the salad sit for 10 minutes at room temperature so the flavors blend and the onion softens slightly.
Add the feta cubes and chopped parsley or dill. Fold gently once or twice, just enough to combine, so the feta doesn't break apart.
Taste and adjust salt, keeping in mind the feta and olives already add a salty note.
Serve right away, or within 1 hour, spooning any dressing from the bottom of the bowl back over the top.