Okla. Stat. tit. 45 § 749
In surface coal mining where the volume of overburden is large relative to the thickness of the coal deposit and where the operator demonstrates that due to volumetric expansion the amount of overburden and other spoil and waste materials removed in the course of the mining operation is more than sufficient to restore the approximate original contour, the operator shall after restoring the approximate contour, backfill, grade and compact, where advisable, the excess overburden and other spoil and waste materials to attain the lowest grade but not more than the angle of repose, and cover all acid-forming and other toxic materials, in order to achieve an ecologically sound land use compatible with the surrounding region. Such overburden or spoil shall be shaped and graded in such a way as to prevent slides, erosion and water pollution and shall be revegetated in accordance with the requirements of this act.
Okla. Stat. tit. 45, § 749