Oregon Revised Statutes Public Health and Safety § 450.165

(1) Whenever the board deems it expedient or necessary to cause to be constructed sewers, drains or sewage treatment plants, the cost of which, in whole or in part, is to be paid either by the proceeds of the sale of bonds by the district or assessed against the property directly benefited or by both methods in proportion, the board shall retain a registered professional engineer to prepare plans and specifications for the sewers, drains or sewage treatment plants, which plans and specifications shall be filed in the office of the secretary of the district.

(2) The district board may, however, adopt any plans and specifications they see fit, provided the plans have been prepared by a registered professional engineer and have been approved by the Oregon Health Authority and the Environmental Quality Commission.