Wash all vegetables thoroughly. Slice cucumbers in half lengthwise, then cut into half-moons about 1/4-inch thick.
Core and dice the bell peppers into bite-sized pieces. Halve the cherry tomatoes. Thinly slice the red onion.
In a large bowl, combine cucumbers, peppers, tomatoes, onion, and parsley.
In a small jar or bowl, whisk together olive oil, red wine vinegar, lemon juice, Dijon mustard, salt, pepper, and oregano until emulsified.
Pour dressing over the salad and toss gently to coat.
Let sit for 5 minutes before serving to allow flavors to meld. Serve chilled or at room temperature.