Prepare the vegetables: Cut cucumbers in half lengthwise, then slice into 1/4-inch half-moons. Dice tomatoes into bite-sized pieces. Dice bell peppers into similar-sized pieces. Thinly slice red onion. Chop parsley.
Make the dressing: In a small bowl or jar, whisk together olive oil, lemon juice, red wine vinegar, Dijon mustard, minced garlic, salt, black pepper, and dried oregano. Add red pepper flakes if using. Taste and adjust seasoning.
Combine: In a large bowl, toss together the cucumbers, tomatoes, bell peppers, red onion, and parsley. Pour the dressing over the top and toss well to coat all vegetables.
Let it rest: Allow the salad to sit at room temperature for 15 minutes to let the flavors meld.
Serve: Give the salad a final toss, then transfer to a serving platter or bowl. Garnish with additional parsley if desired. Serve at room temperature or chilled.