hi,

see ongc generally hires from IITS and top nits. ongc has past years comparatively less vacancy through gate. so if you are currently studying in b.tech petroleum engineering, you should opts for gate exam, if the psus can not be secured, I suggest you to go for m.tech course from top institute like IITS where you would get decent placement as per your branch.

generally vacancy for any psu for any year can not be predicted, it depends on the organization work opportunity, projects availability, seat matrix available for new employee.

nevertheless, if you have attempted gate exam, you may give a shot to careers360 college predictor tool to know about psu chances and probable name of m.tech colleges:

https://engineering.careers360.com/gate-college-predictor?icn=QnA&ici=qna_answer