Okla. Stat. tit. 11 § 37-117
Any municipality owning and operating or proposing to own and operate a system of waterworks to supply the municipality, and the inhabitants thereof with water may provide for its contemplated future water and waterworks requirements, in advance of its immediate needs. For that purpose the governing body may acquire lands and water both within and without the corporate limits of the municipality and within the State of Oklahoma. Such municipalities are vested with the power of eminent domain for that purpose, such power to be exercised in the manner provided by law for the condemnation of lands by municipalities for waterworks purposes. It shall be no defense against the exercise of such power or eminent domain that the municipality is selling and furnishing water to other municipalities or to persons, firms or corporations without the corporate limits of the municipality.
Okla. Stat. tit. 11, § 37-117