As you want to apply online for the recruitment in Oil and Natural gas Corporation (ONGC), I am giving you information about it, check below for the details
I want to tell you that currently there is no recruitment going on in ONGC.
But to apply online you can follow steps given below
Go to ONGC website
Place cursor on Career,
Then click on Notices, the page open look like



Here you will find the information about the recruitment.
Here I found following:

Walk-In-Interview for Engagement of General Duty Medical Officer on Contract Basis for Ahmedabad and Cambay
01 June 2016
Walk-in-Interview for Medical Officers on contract basis at Chennai
25 May 2016
Advertisement for Walk in Interviews for Field Medical Officers and GDMO at Goa
24 May 2016
Walk-In-Interview for Contractual Medical Officers, in ONGC A&AA Basin, Jorhat
20 May 2016
Walk-in-Interviews for the post of Field Medical Officers, GDMO and Specialists, WOU, Mumbai.- Advt 02/2016/DOC/WOU
18 May 2016
Advertisement for conducting Walk in interview for Field Medical Officer
17 May 2016