on campus
On the grounds of a university's or college's campus. Hyphenated if used before a noun. I thought living in the dorms would be fun, but I really didn't like living on campus. The university boasts a huge range of on-campus services and amenities.
on campus
located or being on the grounds of a college or university. Do you live on campus or off campus? I don't think that Lisa is on campus right now.