Can the board add an action item to the agenda at the time of the meeting if the topic was not listed on the original meeting posting notice?
Yes. Oregon law does not require that the actual agenda be posted and does not limit the board to discussing only those topics. The law says that the board must post a list of the topics that it reasonably expects to discuss at the meeting. As long as the board did not know that it was going to discuss the topic at the time of the posting they can add it at the time of the meeting. Doing this repeatedly may raise concern in the "court of public opinion" that the board is not acting openly and transparently so every effort should be made to not have last minute additions and changes.
ORS 192.640 Public notice required; special notice for executive sessions, special or emergency meetings. (1) The governing body of a public body shall provide for and give public notice, reasonably calculated to give actual notice to interested persons including news media which have requested notice, of the time and place for holding regular meetings. The notice shall also include a list of the principal subjects anticipated to be considered at the meeting, but this requirement shall not limit the ability of a governing body to consider additional subjects.