In a large stock pot, place water, broth, peas, ham shank, onion, garlic, oregano, pepper and bay leaf; bring to a boil over high heat; reduce heat to low, cover and simmer 1½ hours.
Remove ham bone/hock/shank; when cool enough to handle, trim meat off bone and return meat to the pot; meanwhile, add celery, carrots and cubed ham.
Return to a bubbly boil; reduce heat to medium-low and continue to simmer uncovered another 2 hours, or longer, until the peas have broken down and the soup has thickened.