Okla. Stat. tit. 60 § 172
No such express trust shall be valid unless created first, by a written instrument subscribed by the grantor or grantors duly acknowledged, as conveyances of real estate are acknowledged, and recorded in the office of the county clerk of each county wherein is situated any real estate conveyed to such trustee, as well as in the county where the principal property is located or business conducted; or, second, by a will duly executed, as required by the law of the state. Such express trusts shall be limited in the duration thereof either to a definite period of not to exceed twenty-one (21) years, or to the period of the life or lives of the beneficiary or beneficiaries thereof in being at the time of the creation of the trust. The instrument creating the trust shall specify the period of duration thereof within the limitations herein provided. When such express trust has originally been created for a definite term of years by a writing other than a will, the time of the existence of such express trust may be extended for a period of not exceeding twenty-one (21) years at any one time, by a written instrument subscribed by all beneficiaries of such express trust, duly acknowledged as are conveyances of real estate, and recorded in the office of the county clerk of the county where is located the principal office of said trust, and in each county where is situated any real estate owned by such express trust. Provided the provisions of this section shall be applicable and limited to business trusts and shall have no application to personal trusts.
Okla. Stat. tit. 60, § 172