The visitors seemed set to grab all three points after Florian Thauvin and Morgan Sanson gave them the edge at the break despite Kevin Monnet-Paquet’s equaliser.
But mid-table St Etienne's Slovenia striker Robert Beric levelled again 15 minutes from time.
Marseille have 52 points from 25 games, 10 points behind leaders Paris Saint-Germain who have a game in hand.
Third-placed Monaco are two points behind OM and will leapfrog them with a win at Angers on Saturday evening (Sunday morning AEDT).
Thauvin volleyed home after four minutes as the visitors got off to a flying start at Geoffroy Guichard.
But St Etienne levelled five minutes later when Monnet-Paquet beat goalkeeper Steve Mandanda from close range.
OM, however, were in front again in the 20th minute, Sanson latching onto a cross from Hikori Sakai.
St Etienne stayed focused though and were rewarded in the 75th when Beric chested home from Jonathan Bamba’s cross.