Okla. Stat. tit. 11 § 37-120
Any municipality owning or operating its own waterworks plant and every municipality engaged in the distribution of water may extend its lines beyond the corporate limits of such municipality and may acquire, construct, own, maintain and operate all necessary lines, apparatus and equipment, and acquire rights of way, and do all things necessary and proper in carrying on the business and furnishing water to any person, firm, corporation or other municipality beyond the corporate limits of the municipality, to the same extent as it may now do within the corporate limits of the municipality. All water so sold and furnished to persons, firms, corporations and other municipalities beyond and without the corporate limits of the municipality so selling and furnishing the same shall be sold and furnished only under written contracts upon such terms and conditions as may be agreed upon by negotiation between the municipality and such persons, firms, corporations or other municipalities. Nothing herein contained shall be construed to impose any duty or obligation upon any municipality to sell or furnish water to any person, firm, corporation or municipality beyond its corporate limits except as the municipality may expressly undertake by the terms of the written contract.
Okla. Stat. tit. 11, § 37-120