Prepare the Vegetables:
Wash the cucumbers, tomatoes, and onion. Slice the cucumbers into rounds, chop or slice the tomatoes, and thinly slice the red onion. Place them in a large mixing bowl.
Make the Dressing:
In a small bowl, whisk together olive oil, red wine vinegar, oregano, salt, and pepper. Adjust seasoning to taste.
Combine the Salad:
Pour the dressing over the vegetables in the mixing bowl. Toss gently to coat all the ingredients evenly.
Garnish and Serve:
If desired, garnish with chopped fresh parsley or basil. Serve immediately, or refrigerate for 20–30 minutes for the flavors to meld.