I think most of the LEO's are going to be pretty relaxed about this law. I know of a couple times people got stopped for it and one of them was in the rain because he was throwing rooster tails up behind him. You never know if you are going to find the guy that is willing to throw the book at you or just let it go with maybe a warning or something.
Oklahoma Statutes Citationized
Title 47. Motor Vehicles
Chapter 12 - Equipment of Vehicles
Article Article 4 - Other Equipment
Section 12-405.3 - Vehicles Without Fenders Over Rearmost Wheels - Rubber Or Fabric Apron Required
All vehicles or combination of vehicles operating on the highways, except animal-drawn vehicles, not equipped with fenders over the rearmost wheels shall have attached thereto a rubber or fabric apron directly behind the rearmost wheels, and hanging perpendicular from the body of the vehicle. The apron shall be of such a size as to prevent the bulk of the water or any other substance picked up from the roadway from being thrown from the rear wheels of the vehicle or combination of vehicles at tangents exceeding twenty-two and one half (22 1/2) degrees measured from the road surface. The provisions of this subsection shall not apply to a farm tractor moving over the state highway system at a speed less than twenty (20) miles per hour.