Oregon Constitution Art. XI-A § 4

There shall be levied each year, at the same time and in the same manner that other taxes are levied, a tax upon all property in the state of Oregon not exempt from taxation, not to exceed two (2) mills on each dollar valuation, to provide for the payment of principal and interest of the bonds authorized to be issued by this article. The two (2) mills additional tax herein provided for hereby is specifically authorized and said tax levy hereby authorized shall be in addition to all other taxes which may be levied according to law.