ORS § 305.575
In an appeal to the Oregon Tax Court from an assessment made under ORS 305.265, the tax court has jurisdiction to determine the correct amount of deficiency, even if the amount so determined is greater or less than the amount of the assessment determined by the Department of Revenue, and even if determined upon grounds other or different from those asserted by the department, provided that claim for such additional tax on other or different grounds is asserted by the department before or at the hearing or any rehearing of the case before the tax court. In the event such other or different grounds are asserted by the department, the opposing party shall be allowed additional time, not less than 10 days, within which to amend or otherwise plead thereto, which additional time, however, may be waived by stipulation of the parties. The order of the tax court shall be sufficient for the collection by the department of the entire amount found by the court to be owing and due.
ORS 305.575