In SEVP's OPT 2010 Policy Guidance, Section 7.2.1 (p. 17-18) states, in part:
“Unpaid employment. A student may work as a volunteer or unpaid intern, where this practice does not violate any labor laws. The work must be at least 20 hours per week for a student on post-completion OPT. A student must be able to provide evidence acquired from the student’s employer to verify that the student worked at least 20 hours per week during the period of employment.”
See: http://www.kidambi.com/resources/OPT_Volunteer.pdf