ORS § 223.395
If the initial assessment has been made on the basis of estimated cost, and upon the completion of the work the cost is found to be greater than the estimated cost, the governing body may make a deficit assessment for the additional cost. Proposed assessments upon the respective lots within the assessment district for the proportionate share of the deficit shall be made; and notices shall be sent; opportunity for objections shall be given; such objections shall be considered; and determination of the assessment against each particular lot, block or parcel of land shall be made as in the case of the initial assessment; and the deficit assessment spread by ordinance. If assessments have been made on the basis of estimated cost, and upon completion the cost is found to be less than the estimated cost, provision shall be made for refund of the excess or overplus.
ORS 223.395