ORS § 275.105
If any county has bid in and acquired for taxes and has received a deed for not less than 90 percent of the number of the lots in any addition or subdivision or plat, and if it considers it wise so to do, the governing body of the county shall, by order duly made and entered, authorize the purchase of such remaining lots from the owners or may exchange for the lots other lots owned by such county. Upon acquiring title to all the lots in any addition or subdivision or plat, it may enter an order vacating the whole of such addition, subdivision or plat. If any remaining lots are purchased by the county pursuant to this section, the purchase price of the lots shall not be greater than the real market value of the lots, and if other lots are exchanged for the remaining lots, those lots shall be accepted in full payment of the purchase price of the lots for which they are exchanged.
ORS 275.105