Oregon Revised Statutes Insurance § 746.145

(1) Notwithstanding ORS 737.600, but subject to all other rate filing requirements of ORS chapter 737, an insurer may combine for dividend purposes the experience of a group of employers covered for workers' compensation insurance by the insurer, subject to applicable rules adopted by the Director of the Department of Consumer and Business Services, if:

(2) This section does not apply to an organization of employers for which organization a workers' compensation policy was lawfully issued before October 4, 1977. The policy required by ORS 656.419 shall contain for each employer covered thereby the information required by ORS 656.419 (2). When an employer becomes an insured member of the organization the insurer shall, within 30 days after the date insured membership commenced, file a notice thereof with the director.