I'm graduating senior(F1 visa) in this December. I'm waiting for my OPT card. Can I work in between that? Once I get my OPT can I apply for H1B on year 2016 or will I have to complete H1B? What other legal things I need to be aware of?
You can't start working until you have your OPT work card in hand. There are circumstances where someone who hasn't yet graduated can be sponsored in the H-1B lottery, but this essentially requires the student to have completed all requirements for the degree by the time of filing (in April).
