Okla. Stat. tit. 60 § 175.56
When the declaration or agreement of an express trust provides for any of the property held in trust to be distributed to a beneficiary related by blood to the grantor or to a grantor of the trust, and the beneficiary is living at the time the trust is created but dies before the time for distribution of the trust leaving one or more lineal descendants who are living at the time for distribution of the trust, and no provision is made in the trust declaration or agreement for disposition of the property in the event that the beneficiary is not living at the time for distribution of the trust, the beneficiary's lineal descendants take the share of the trust property so given to the beneficiary in the trust declaration or agreement, by right of representation, in the same manner as the beneficiary would have done had he been living at the time for distribution of the trust.
Okla. Stat. tit. 60, § 175.56