Heat the olive oil in a large pot over medium heat. Add the beef stew meat and cook until browned on all sides, about 5 minutes.
Add the onion, carrots, and garlic to the pot. Cook for 4 minutes until the vegetables start to soften.
Stir in the dried oregano, diced tomatoes, and beef broth. Bring the mixture to a boil.
Add the rinsed lentils and reduce the heat to low. Cover and simmer for 35 minutes, stirring occasionally.
Stir in the kalamata olives and cook uncovered for 5 more minutes until the lentils are tender. Season with salt and black pepper.
Ladle the stew into bowls and top each serving with crumbled feta cheese.