Oregon Revised Statutes Oregon Vehicle Code § 815.205

(1) Motor vehicles registered as farm vehicles under ORS 805.300 are not subject to the limits on visible emissions.

(2) Vehicles of special interest and antique vehicles are not subject to the limits on visible emissions if the vehicles are maintained as a collectors' item and used for exhibitions, parades, club activities and similar uses, but not used primarily for the transportation of persons or property.

(3) The visible emission limits apply only in counties having a population over 50,000 according to the 1970 federal decennial census that are located west of the summit of the Cascade Mountains. The summit of the Cascade Mountains is determined for purposes of this subsection by the line beginning at the intersection of the northern boundary of the State of Oregon and the western boundary of Wasco County, thence southerly along the western boundaries of the counties of Wasco, Jefferson, Deschutes and Klamath to the southern boundary of the State of Oregon.