That is handled in the Federal section, and then gets transferred to the OR forms from there.
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Go back to teh Federal section and edit any 1099-INT or 1099-DIV that had Muni bond dividends reported on it.
When you entered that Muni bond interest/dividends....either in box 12 of a 1099-DIV form, or in box 8 of a 1099-INT form.....ther is a follow-up page that is similar to the following (picture from Online software). You can tag it all as being "Mutiple States", or break out just the OR Muni bond portion from all the rest of the states as follows (NC resident example):
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