The hosts raced into a 2-0 lead in the first half through Gael Danic and Anthony Le Tallec before Marseille's dismal afternoon took a further nosedive when goalkeeper Steve Mandanda inexplicably rolled the ball out to VA's Foued Kadir, who had a simple task to fire home.
Le Tallec added his second after the break before Jordan Ayew bagged a late consolation for the beleaguered visitors.
The defeat ends a run of six straight wins at the start of the season for OM, who would have seen their lead at the summit trimmed to a point if Lyon had defeated Bordeaux.
But instead Les Gones blinked first in the battle of two unbeaten sides as goals from Benoit Tremoulinas and Cheick Diabate gave Bordeaux victory at the Stade Gerland.
Remi Garde's men were left to rue a disallowed goal from Alexandre Lacazette as they strived to peg Bordeaux back following Benoit Tremoulinas' deflected opener.
But try as they might, Lyon could not break down a determined Cedric Carrasso in the visiting goal, with Diabate confirming Les Girondins' ascent to fifth place in the league table.
Elsewhere, Kossi Agassa denied St Etienne all three points at the death as plucky Reims took a point from a goalless draw.
Visiting goalkeeper Agassa endured some shaky moments as Les Verts dominated the creative play - ex-Leeds man Max Gradel at the heart of the performance - while opposite number Stephane Ruffier also remained resolute.
But it was Agassa who repelled Brandao's acrobatic effort deep into stoppage time to extend Reims' unbeaten run to a fifth Ligue 1 game, also leaving St Etienne without a league win in four attempts.
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Chelsea won the Europa League on Wednesday. The Blues defeated Benfica 2-1 on injury time goal from Ivanovic.
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