Peel and slice the onion, peel and finely slice the garlic, and quarter the potatoes. If using fresh, quarter the tomatoes. Destone the olives. Heat the oil in a large saucepan over a medium heat and sauté the onion, oregano and garlic for 4 to 5 minutes, or until softened. Season generously. Add the potatoes, tomatoes (breaking them up with a spoon, if using tinned), bay leaves and tomato purée to the pan and stir well. Add enough water to just cover the mixture and then simmer, covered, over a low heat for 20 to 30 minutes. Remove the lid, add the olives and continue to cook, stirring occasionally, for 10 minutes, or until the sauce has thickened and the potatoes are tender. Serve hot or at room temperature with a slab of feta and a sprinkling of oregano and a drizzle of oil, or as a side to grilled poultry.