Hi Chetan,
In India the most Popular Foundation that Conducts Olympiad which is SOF, i am telling you about their criteria.
Sorry to tell you that SOF ( Science Olmypiad Foundation ) accepts only a candidate from school no individual registrations are allowed for the Olmpiad.
The same you may check at the link given below : http://www.sofworld.org/nso/about-nso-exam/how-to-participate
Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile