Oregon Constitution Art. IV § 32

Notwithstanding any other provision of this Constitution, the Legislative Assembly, in any law imposing a tax or taxes on, in respect to or measured by income, may define the income on, in respect to or by which such tax or taxes are imposed or measured, by reference to any provision of the laws of the United States as the same may be or become effective at any time or from time to time, and may prescribe exceptions or modifications to any such provisions. At each regular session the Legislative Assembly shall, and at any special session may, provide for a review of the Oregon laws imposing a tax upon or measured by income, but no such laws shall be amended or repealed except by a legislative Act.