you said that you're graduating this semeseter and also said that you are working on OPT, doesnt make sense. if you are already graduated and you are walking on May and you are already on OPT working full time, tht supposed to be a strong case for your parents to get visa. Working on OPT on your field is great.

For the interview for you parents: they will ask your parents about whr u studying , wht u studying, and what are their purspose of the visit. tell your father to know your major field of study. Try to get a reference letter from your parents govt. office if they work for govt. office. Thts the only questions they ask my parents and they got it. they r with me right now.

hope this helps.