Oregon Revised Statutes Public Health and Safety § 450.860

When in providing sewage disposal or drainage systems for an area within the authority the board determines that any portion of sewage treatment plants or trunk or lateral sewers or drains constructed or acquired, or being constructed or acquired, by the authority, will be used by the area, a fair and equitable portion of the original cost of such plants, sewers or drains and of improvements thereto, less depreciation, shall be charged to the area on an assessment or sewer service charge basis, or any combination thereof. The proceeds of such assessments or charges shall be used by the board, in its discretion, for payment of bonds issued to construct such plants, sewers or drains or for future improvements or additions to or maintenance of such plants, sewers or drains. The determination made under this section shall be made at the hearing of the board in connection with any proposed construction of sewage disposal or drainage systems for the area.