Okla. Stat. tit. 47, § 47-14-106: Trailers and towed vehicles

Every trailer, or semitrailer, shall be equipped with a coupling

device which shall be so designed and constructed that the trailer,

or semitrailer will follow substantially in the path of the vehicle

drawing it without whipping or swerving from side to side. In

addition, every such trailer or semitrailer except a semitrailer

drawn by a truck-tractor type designed to draw or support the front

end of a semitrailer, shall be coupled with stay chains or cables to

the vehicle by which it is being drawn which chains or cable shall

be of sufficient size and strength to prevent parting from the

drawing vehicle should the regular coupling device break or become

otherwise disengaged.