Place oil and bacon in a 6-qt. pot and cook over medium-high heat until crisp, about 6 minutes. Transfer bacon to paper towel with a slotted spoon; set aside. Add onions, celery, carrots, and celery root, season with salt, and cook, stirring occasionally, until soft, about 10 minutes. Stir in flour; cook for 3 minutes.
Tie parsley, thyme, and bay leaves together with kitchen twine; add to pot with peas, ham hocks, and 7 cups water. Bring to a boil over high heat. Reduce heat and simmer, covered, until peas are very tender, about 1 hour. Remove from heat. Discard herbs. Transfer hocks to a plate to let cool; pull off and chop the meat; discard fat, skin, and bones. Stir meat into soup, season with salt and pepper, and ladle soup into bowls.