Oregon Revised Statutes Commercial Transactions § 83.130

(A) The service charge earned to the date of prepayment, computed by applying the effective rate on the contract to the actual principal balances outstanding, for the periods of time the balances were actually outstanding. In determining the effective rate, the holder may apply to the scheduled payments the actuarial method by which each scheduled payment is applied first to the accrued and unpaid service charges and any amount remaining is applied to the reduction of the principal balance.

(B) The service charge earned to the installment due date nearest the date of prepayment, computed by applying the effective rate on the contract to the actual principal balances outstanding, for the periods of time the balances were actually outstanding. For purposes of rebate computations under this subparagraph, the installment due date preceding the date of prepayment shall be considered to be nearest if prepayment occurs 15 days or less after that installment date. If prepayment occurs more than 15 days after the preceding installment due date, the next succeeding installment due date shall be considered to be nearest to the date of prepayment. In determining the effective rate, the seller may apply to the scheduled payments the actuarial method, by which each scheduled payment is applied first to the accrued and unpaid service charges and any amount remaining is applied to reduction of the principal balance.