Okla. Stat. tit. 60, § 175.402

DISTRIBUTION FROM TRUST OR ESTATE A trustee shall allocate to income an amount received as a distribution of income from a trust or an estate in which the trust has an interest other than a purchased interest, and shall allocate to principal an amount received as a distribution of principal from such a trust or estate. If a trustee purchases an interest in a trust that is an investment entity, or a decedent or donor transfers an interest in such a trust to a trustee, Section 10 or 24 of this act applies to a receipt from the trust.