In a large skillet, saute the celery, onion, green pepper and carrot in oil until tender. Stir in flour until blended; gradually add broth. Bring to a boil. Cook and stir for 2 minutes or until thickened., Transfer to a 3-qt. slow cooker. Stir in the sausage, tomatoes, oregano, thyme and cayenne. Cover and cook on low for 4-5 hours or until heated through. Serve with rice. Freeze Option: Freeze soup and rice, if desired, in freezer containers. Thaw in the refrigerator overnight. Transfer to a saucepan. Cover and cook over medium heat until heated through, adding a little water if necessary. Serve gumbo with rice.