Place lentils in a large pot with enough water to cover be a few inches. Bring to a boil, and cook until lentils are tender, about 15 minutes. Drain and rinse with cold water in a colander to cool. Let stand for a few minutes to drain well.
In a large bowl, whisk together the olive oil and lemon juice. Add the red pepper, onion and green olives, and toss to blend. Refrigerate for about 2 hours before serving to blend the flavors. This salad tastes better the longer it sits.