From the HIO website (http://www.hio.harvard.edu/campus-employment):
Work on the Harvard campus is permissible as soon as students are in F-1 or J-1 status in the U.S., though no sooner than 30 days prior to the start of classes. Acceptable employment includes: work required by a scholarship, assistantship or fellowship, work in the libraries, computer centers, administrative offices, and work performed on Harvard's premises for an outside contractor or firm as long as the employer provides direct services to students (for example, work for a food service company at Harvard). Employment is limited to 20 hours per week while school is in session, but may be full-time during Harvard's vacation periods. J-1 students are required to report their on-campus employment to the HIO once every year.
On-Campus Employment at an Off-Campus Location for F-1 Students
USCIS has expanded the definition of on-campus employment to include jobs with employers that are educationally affiliated with Harvard University, provided that the job is associated with Harvard's curriculum or related to contractually-funded research projects and provided that the job is an integral part of students' academic programs (for example, research work at one of Harvard's affiliated hospitals may be acceptable for students in the sciences). Employment is limited to 20 hours per week while school is in session, but may be full-time during Harvard's vacation periods. Final permission is granted by the student's HIO advisor.