Oregon Constitution Art. IV § 10

(1) The Legislative Assembly shall hold annual sessions at the Capitol of the State. Each session must begin on the day designated by law as the first day of the session. Except as provided in subsection (3) of this section:

(2) The Legislative Assembly may hold an organizational session that is not subject to the limits of subsection (1) of this section for the purposes of introducing measures and performing the duties and effecting the organization described in sections 11 and 12 of this Article. The Legislative Assembly may not undertake final consideration of a measure or reconsideration of a measure following a gubernatorial veto when convened in an organizational session.

(3) A regular session, as described in subsection (1) of this section, may be extended for a period of five calendar days by the affirmative vote of two-thirds of the members of each house. A session may be extended more than once. An extension must begin on the first calendar day after the end of the immediately preceding session or extension except that if the first calendar day is a Sunday, the extension may begin on the next Monday.