In a 5-quart slow cooker, combine onion, oregano, bay leaves, chipotle, adobo sauce, tomatoes (and puree), salt, and pepper. Add pork; toss to coat with sauce.
Cover; cook on high setting until meat is pull-apart tender, about 6 hours (do not uncover while cooking).
Transfer meat to a large bowl; shred with forks, discarding any gristle. Return meat to pot; toss with sauce.
To serve, discard bay leaves; if desired, garnish with tortillas and cheddar or sour cream.