Generally, additions are items not taxed by the federal government but taxed by Oregon. Additions increase the income taxed by Oregon.
Interest and Dividends on State and Local Government Bonds Outside of Oregon
You must add to Oregon income any interest and dividends you received from state and local governments outside Oregon. You don't pay federal tax on this interest, but you do pay Oregon tax.
See Form 40 Instructions for a full list of other Additions.