Hello immigration Gurus of Sajha,
Here's my scenario:-
1) it's my final semester
2) I will graduate in May
3) I'll apply for OPT 1 or 2 months ahead of Graduation
4) my visa expires June 30
5) I want to go to Germany for a seminar related to my studies from June 1 to June 7.
6) After I come back, I want to find a job and work on OPT.
7) I have traveled to Nepal twice before. No immigration problems.

Can I travel to Germany and still enter to the United States without any hassle? If not, what should I do to make it a safe travel? Please give me your opinion.
Thank you!

I had friend who came back while he had 10 days left on VISA, So i believe if you have the VISA valid then you could. OPT, they won't start till you are fully done and certified by adviser that you are done completing your course which would be end of the semester before they even process your OPT until you have special cases like getting a job way before course completing (specially done with course but not the entire semester) and your adviser filed for early OPT...

Your I-20 expires the day you graduate and therefore you will not be allowed to re-enter U.S. even if you have an unexpired visa. If I were in your shoes, here's what I'd do:
- start looking for jobs from today
- apply for OPT 2.5 to 3 months before the day of graduation
- get hired or at least get a job offer letter
- then go to Germany

Keep in mind: travelling while your OPT application is pending is risky; even more so for you because your visa is about to expire
so the  bottomline is, don't leave U.S. for any reason until you get hired or at least until you get an offer letter from your employer

If you have OPT card on your hand while returning to USA, you can go to abroad (Germany) and reenter to US. Because only you graduate you should be in OPT to maintain status. Even if you have visa on passport, you are not in status as a student. Hope it helps.

 It's not a good idea to travel outside the US while on OPT.

However I've found a link that might be pretty helpful for the person in your shoes...read thoroughly.

http://www.oip.lsu.edu/iso/pdfs/Can%20I%20travel%20outside%20the%20US%20while%20on%20OPT.pdf

 travelling while you're on OPT is perfectly fine. I'm doing my OPT and recently went home for a month and came back without any problems. And guess what, my POE was the one everyone's scared of - Chicago. नबिराउनु नडराउनु!
if you don't have fake degrees/trainings/resumes like some people do (just check a recent thread about a student on OPT getting deported), there's nothing to worry about.