If you had installment sales where you were required to pay interest on the deferred tax liability for federal purposes, you must also compute interest for Oregon. The amount due for Oregon is computed the same as the federal amount. The interest rate is 0.3333% per month for 2015.
An installment sale is a sale of property at a gain where at least one payment is to be received after the tax year in which the sale occurs. You are required to report the sale under the installment method unless you "elect out" on or before the due date for filing your tax return for the year of the sale. If you elect out, you report all the gain as income in the year of the sale. Installment sale rules do not apply to losses. You cannot use the installment method to report gain from the sale of inventory or stocks and securities traded on an established securities market.