1. Heat oil in a large Dutch oven over medium heat. Add celery, carrot, onion, garlic, oregano, salt, and pepper; cook 5 minutes.
2. Add the water and lentils. Simmer, partially covered, 15 minutes.
3. With a hand blender or potato masher, puree soup until semi-smooth and thick.
4. Drizzle with lemon juice; serve with toasted pita triangles.