No Object Certificate

[fordad]

HI,

I am a student in US, who is about to graduate in May 2010
I have sent an invitation letter to my dad from my university.
My dad is a Government employee in Andhra Pradesh, he has a passport
Now my questions are:
1)when should he take the NOC for visiting US, is it before stamping or after stamping.
2) what are the documents he need to submit for applying for the NOC.
3) what are the documents he need to show to the VO


Thanks and Regards