ORS § 317.950
If the Department of Revenue finds that unrelated business taxable income, or any portion thereof, has not been assessed, it may, at any time within three years after the return was filed, or in case no return was filed within five years from the time the return should have been filed, compute the tax and give notice to the corporation of the amount due, including penalty and interest thereon. These limitations to the assessment of such tax or additional tax, including penalty and interest thereon, do not apply to the assessment of additional taxes, and penalty and interest thereon, upon false or fraudulent returns or in cases where with a fraudulent intent no return has been filed. ORS 314.410 is also applicable to the extent that it is not inconsistent with the provisions of this section.
ORS 317.950