As far as the OPT rule is concerned, u have to work in ur field of Study. And on 12 months OPT Volunteer can be 20 hr/week or 40 hr/week without any pay whatsoever..Interesting is it contradicts the labor law, if u r working u have to be paid..But on 12 months OPT Working without pay is fine as long as u r working in Field of Study..For eg: Working as an Intern in some ABC comapny without getting paid...But in some cases like EE,Electronics there is no thin redline to tell whether u have to work for a EE or Electronics company..PPL r working in Core IT field..But "U definitely can't work as a Volunteer Programmer if u had a degree in Plant Sciences"