As you wants to apply for job in PSU's like ONGC,NTPC,BHEL,IOCL , then you have to appear in a written test exam first, which is conducted by the PSU's individually..
But to apply for a job in IOCL you must have to appear in GATE exam, and you have to perform well in GATE, because IOCL does not conduct any written test exam , they hire students on the basis of GATE score..
The other PSU's hire students on the basis of the score in the written test which is conducted by them self..
If you have a good gate score then you can apply for IOCL..
but for the other PSU's you have to go through the written test...