Okla. Stat. tit. 60, § 60-34
Contingent remainder in fee, on prior remainder
A contingent remainder in fee may be created on a prior
remainder in fee, to take effect in the event that the persons to
whom the first remainder is limited die under the age of twenty-one
(21) years, or upon any other contingency by which the estate of
such persons may be determined before they attain majority.