Corporate Excise Tax—Single Sales Factor. Oregon Corporation Excise Tax Form OR-20 Instructions Table of contents Important reminders ... secure access to tools for managing your Oregon tax account. • Make payments. If this happens, you must file an OR-20-INS Oregon return, check the "Amended" box, and attach a copy of the federal or Insurance Division audit report. The 2017 Oregon Legislature decided that, starting with the 2018 tax year, corporate excise taxpayers must use market-based sourcing principles rather than cost of performance for receipts other than those from the sale of tangible personal property. You must notify us by amending your Oregon return if the IRS changes your federal return or the Insurance Division changes your fire marshal tax or retaliatory tax, and the change affects your computation of Oregon excise tax. With Revenue Online, you may: • View your tax account. To amend your Oregon S corporation excise or income tax return, use the form for the tax year you’re amending and check the "Amended" box on the front of the return. See the Form OR-20-S instructions for more information. The tax on subchapter C corporations doing business in the state is the greater of: minimum, ranging from $150 up to $100,000, based on a schedule in relation to Oregon sales (approximating 0.1% of sales), which is not reduced by any income tax credit, or • View correspondence we sent you. • Check the status of your refund.