Oregon Revised Statutes Water Resources: Irrigation, Drainage, Flood Control, Reclamation § 552.138

(1) No final order creating a district shall be set aside, or annulled upon appeal or review, on account of any defect or irregularity in the petition asking for organization of the district, or notice of hearings thereon, which does not materially affect the substantial rights of an interested party.

(2) No proceeding may be maintained contesting the validity of the creation of a district unless instituted within 90 days after the entry of the final order of the county board.