Cut onions into very thin slices and separate into rings. Soak onions in ice water for one hour. Set aside. Combine flour, salt, pepper, oregano and Cajun spice in a large bowl. Heat oil. Remove onions from ice water and dry thoroughly with paper towels. Dredge in coating, then fry until brown. Drain on paper towels. Season with salt and serve.
The oil temperature will drop when the onions are added, then rise as the onions cook; regulate the heat accordingly.