Brown chicken in hot oil in a large stock pot. Remove chicken and set aside, reserving pan drippings in stock pot.
Add flour to pan drippings; cook over low heat, stirring constantly, 50 minutes or until roux is the color of a copper penny. Add chopped onion, and cook 1 minute, stirring constantly.
Gradually add 3 quarts water to roux, stirring until well blended. Add reserved chicken, salt, and pepper. Bring mixture to a boil. Reduce heat, and simmer, uncovered, 2 hours, stirring occasionally.
Add oysters and simmer, uncovered, 5 minutes; stir in parsley and onion tops and cook an additional 5 minutes. Remove from heat.
Stir in filé powder to thicken gumbo. Ladle gumbo over hot cooked rice in individual serving bowls and serve immediately.