Some candidates choose to not be affiliated with any political party. We call these nonaffiliated candidates, although they are commonly referred to by voters and the media as independent.*
Primary elections are held by major parties to determine their nominee for the General Election in November. Offices like Governor, U.S. Senator, or State Representative are examples of partisan offices that have primary elections. Since nonaffiliated candidates are not seeking a party nomination, they cannot participate in the primary.
Some offices are also nonpartisan, which means no candidates run with partisan labels. For example, judicial races are nonpartisan races in Oregon. These races do appear on primary ballots.
*Not to be confused with the Independent Party, which is a recognized minor political party in Oregon.