Add olive oil to a soup pot and sauté the onion, carrot, and celery until the onion is translucent; about 2 minutes.
Stir in the water or broth, rinsed lentils, potato, bay leaves, dried oregano, and salt. Bring to a simmer and cook until the lentils are very soft; about 30 to 40 minutes.
Stir in the vinegar and serve topped with fresh parsley and feta if desired.