Students are required to complete at least one major to graduate from the UO. You can read about the various majors in the list of majors and minors. You should declare a major when you are ready. However, it is a good idea to declare it by the end of winter term during sophomore year in order to avoid adding extra time and expense to your college education.
Keep in mind that many majors require the completion of core requirements. For example, science students, though undeclared, must take mathematics and, usually, general chemistry in the freshman year. Postponing foundation classes can extend the time it takes to earn a degree in some majors.