Heat oil in a heavy stock pot over medium heat. Stir in flour and cook, stirring constantly, to form roux. Mixture should be a dark caramel color; if you burn it, toss it out and start over. Remove from heat and slowly whisk in water; add onion, garlic, parsley, celery, salt, black pepper, okra, chicken, and sausage. Simmer for 2 hours or until chicken is tender and gumbo is thickened; stir in crawfish tails, if using, for the last 10 minutes. Serve over hot rice and pass filé at the table; Cajuns are going to add any of the following to the bowl: a hard boiled egg, a roasted sweet potato, or a scoop of potato salad.