OPT is another exception to the F-1 no-working rule. It is allowed only after completing the first academic year. OPT activity must be related to your field of study. You must apply to the United States Citizenship and Immigration Services for an employment authorization card for OPT purposes.
OPT comes in two different forms.
Pre-completion OPT- which can be used during your vacation and during school. However, during the school year, pre-completion OPT cannot be for more than twenty hours per week.
Post-completion OPT- Post-completion OPT may be obtained after you complete all of the course requirements in your course of study, with the exception of your thesis or dissertation.
Regardless of whether you use pre-completion OPT or post-completion OPT, you can only have a total of twelve months of OPT. Please note that you cannot have more than 90 days of unemployment during this initial OPT period.