Brazilian talisman Jose Ramirez Barreto came to Mohun Bagan's rescue as he found the winner in extra-time to edge past a fighting ONGC 2-1 in the Federation Cup in Ludhiana on Thursday.
The defending champions will now face Viva Kerala, who upset Mumbai's Air India by the same margin, in the quarter-final on Sunday.
Bagan went ahead through an own goal by Gulidum Rongmoi in the 32nd minute before Barreto settled the matter in the 107th. Jatin Singh Bisht scored for ONGC in the 83rd.
Barreto provided a huge sigh of relief for his team when he headed home an L James Singh free-kick to send Bagan into the last eight, a conclusion which seemed almost foregone before the match, without anticipating the determination shown by the PSU team.
For all of Mohun Bagan's dominance in the first half, it needed a defensive lapse in the 32nd minute for the maroon brigade to get ahead.
A corner kick from the left flank by Lalkamal Bhowmick found ONGC's Gulidum Rongmoi at the near post, who in his haste to clear the low cross sliced it into his own net past a hapless Rajat Ghosh Dastidar.
The custodian was the one to be blamed for the two successive corners as he failed to control an innocuous shot from distance and let the ball roll over the backline.