Oregon Revised Statutes Revenue and Taxation § 310.110

(1) If a taxing district lies in two or more counties, and the district certifies an item of ad valorem property tax as an amount in the notice required under ORS 310.060, the amount certified by the district shall be apportioned on the basis of the assessed value used to compute the tax rate for the current tax year, in the proportion that the assessed value of the part of the district lying in each county bears to the assessed value of the whole district. However, if a boundary change affecting the district becomes effective as to the levy being apportioned, an adjustment of the assessed value shall be made so as to reflect the boundary change.

(2) Any assessor who is unable to certify the current assessed value for any joint district lying partially in the county by September 25 shall, with the cooperation of the Department of Revenue, estimate as closely as practicable the assessed value of that district for the purpose of apportioning the ad valorem property taxes of the joint district in the current year as equitably as is possible. The estimate shall be completed and certified to the assessor or assessors of the other counties on the fifth business day after September 25 and shall be used as the basis for the apportionment required by this section.