Let beans soak for about 4 hours before starting to cook. Rinse beans thoroughly. Slice onion, green pepper, and potato to small chunks. Combine beans, water, ham hock, onion, green pepper, potato, oregano, salt and pepper. Bring water to boil. Turn heat down and simmer for 2 hours. Add tomato sauce and simmer for another hour. Bean should be tender if not simmer for another hour.