Mohun Bagan were winless from three matches but the return of Nigerian forward Odafa Okolie from a three-match suspension revived their sagging morale as he combined well with Tolgay Ozbey up front to earn three points for his side.
Bagan's win also ended ONGC's eight-match unbeaten run.
Ozbey (10th) opened the scoring from an Odafa assistance, while the Australian set up for Jewel Raja (63rd) to give a 2-0 lead.
Odafa (72nd) got into the act to complete the rout as the green-and-maroon brigade brought smile back on their supporters ahead of the return leg derby against East Bengal on February 9.
Sohrabuddin Mollick (83rd) reduced the margin for ONGC but could not make further inroads to suffer their first defeat in two months.
The Delhi-based team had lost last on December 1 last year when they were thrashed 1-4 by Pailan Arrows in a round nine fixture.
Mohun Bagan's struggling goalkeeper Shilton Paul stood out under the bar as he thwarted ONGC's attacks on more than one occasions as they won after one loss and two draws since their return from the ban.
It was not a perfect match for the Karim Bencherifa's men as there were a lot of mispasses, especially Manish Mathani who failed to keep the ball on many occasions, but the return of Odafa made all the difference.
The Nigerian combined well with Ozbey giving the fans a lot of hope ahead of the big derby as Mohun Bagan got it going immediately after the start.
In a perfect coordination, Odafa set it up from the right for Ozbey to do the rest as the latter's powerful right-footer just at the top of the ONGC box beat goalkeeper Rehnash T P.
Mohun Bagan held on the lead with Shilton doing a fine job as they made it 2-0 after the changeover when Ozbey's centre from left found an unmarked Jewel who scored easily.
Odafa took the ball from the midfield as he finished well with a solitary effort.
ONGC got the only goal when Mollick got the ball after Shilton failed to clear but it was not enough as Mohun Bagan watched out the remainder seven-odd minutes for their first win after they resumed I-League from zero point.
Battling to avoid relegation, Mohun Bagan now have five points from 14 matches at the bottom spot, while ONGC remained on 21.