Heat the olive oil in a large pot over medium-high heat. Add the lamb cubes and brown them on all sides, about 6 minutes total. Remove the lamb and set it aside.
Lower the heat to medium. Add the onion to the same pot and cook until it softens, about 4 minutes. Stir in the garlic and cook for 1 minute more.
Return the lamb to the pot. Add the barley, diced tomatoes with their juices, beef broth, salt, and pepper. Stir to combine.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 45 minutes, stirring occasionally.
Stir in the kalamata olives and chopped oregano. Continue simmering uncovered for 10 more minutes until the barley is tender and the stew has thickened slightly.
Taste and adjust salt if needed before serving.