Okla. Stat. tit. 45 § 750
The operator shall remove the topsoil from the land in a separate layer, replace it on the backfill area or if not utilized immediately, segregate it in a separate pile from other spoil. When the topsoil is not replaced on a backfill area within a time short enough to avoid deterioration of the topsoil, the operator shall maintain a successful cover by quick-growing plants or other means so that the topsoil is preserved from wind and water erosion, remains free of any contamination by other acid or toxic material, and is in a usable condition for sustaining vegetation when restored during reclamation. If the topsoil is of insufficient quantity or of poor quality for sustaining vegetation, or if other strata can be shown to be more suitable for vegetation requirements, then the operator shall remove, segregate and preserve in a like manner such other strata which is best able to support vegetation.
Okla. Stat. tit. 45, § 750