ORS § 541.080
In any suit commenced for the protection of rights to water acquired under the provisions of the Act of 1891, pages 52 to 60, Oregon Laws 1891, the plaintiff may make any or all persons who have diverted water from the same stream or source parties to the suit, and the court may in one decree determine the relative priorities and rights of all parties to the suit. Any person claiming a right on the stream or source, not made a party to the suit, may become such on application to the court, when it is made to appear that the person is interested in the result, and may have the right of the person determined. The court may at any stage, on its own motion, require any persons having or claiming rights to water on the stream or source, to be brought in and made parties, when it appears that a complete determination of the issue involved cannot be made without their presence.
ORS 541.080