Okla. Stat. tit. 60, § 60-172

duration - Extension of term.

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.