In a mixing bowl, combine the ground beef, oregano, cumin, salt and pepper.
In a frying pan, heat the olive oil. Sauté the onions, green pepper, and garlic until soft.
Add the ground beef mixture, beef stock, and tomato sauce. Cover and cook over medium-low heat for 15 minutes.
Add the diced potato. Cover and cook another 15 minutes, or until the potatoes are tender.
Remove the cover. Add the olives and cook uncovered 15 minutes or until the liquid is fully evaporated, but the meat is still moist.
Serve the picadillo warm with rice or let cool and use as a filling for empanadas.