It's how Oregon calculates Oregon nonresident tax. If you file jointly in Oregon, your spouse's income is factored in to determine Oregon tax. Oregon pretends that all of your joint income is taxable in Oregon, and then prorates the tax to the amount of income actually earned in Oregon. Sometimes filing jointly can lower the tax liability, and sometimes it raises it.
But Oregon gives you the option to file separately, even though you file jointly in Oregon. You can test both ways (joint or separate) and then choose the filing status that results in the largest tax refund or least amount due.
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