No way! It's not just risky to go to nepal on OPT, it's for sure that you can't re-enter US with OPT. OPT: Optional Practical Training... get it? Optional. OPT is not a requirement or part of your study for which you got your F-1 visa. So, you can't get student visa for OPT. Once you go outside US, you will have to apply for visa to re-enter. You have no ground to apply for student visa under OPT. You will have to get another i-20 to apply for student visa again.