Whisk olive oil, lemon juice, mustard, garlic, oregano, salt, and pepper until evenly mixed. Set aside.
Stir cooked lentils, bell pepper, tomatoes, cucumber, red onion, walnuts (if using), olives (if using), parsley, mint, and dill.
Pour dressing over lentil mixture and stir until evenly coated.
Add feta cheese and gently toss until evenly mixed.
