Luiz Araujo's late goal cancelled out Andy Delort's first-half opener for Montpellier as Lille were left on 70 points from 33 games, four ahead of second-placed Paris Saint-Germain who host Saint-Etienne on Sunday.
Montpellier's hopes of clinching a Europa League place faded as they stayed eighth on 47 points.
The hosts piled on the pressure in the second half, but Benjamin Andre's powerful shot from outside the area crashed onto the post in the 57th minute as Montpellier held on.
Omlin pulled another couple of great saves to deny Jonathan Ikone and Jonathan David but there was nothing he could do when Luiz Araujo unleashed a delightful left-footed strike to level with five minutes left.
Lille threatened some more in seven minutes of stoppage time but were not clinical enough to clinch the win.