Cook the quinoa according to package instructions. Let it cool to room temperature.
While the quinoa cools, chop all vegetables: cucumbers, tomatoes, bell peppers, olives, and red onion.
In a large mixing bowl, combine the chopped vegetables with the cooled quinoa.
Add the vinaigrette ingredients—olive oil, lemon juice, red wine vinegar, oregano—directly into the bowl.
Season the salad with salt and freshly ground black pepper to taste.
Toss everything together until well combined.
Serve immediately or chill before serving. Store leftovers in the refrigerator for up to 3 days.