An even contest at the Stade Velodrome swung Arsenal's way in the 65th minute when their French hosts were made to pay for a defensive error, as Jeremy Morel's attempted clearance inside his own box was pounced upon by Walcott, who slammed the ball past Steve Mandanda in goal.
Walcott's first goal of the season was followed by a sixth for the in-form Ramsey, who drove through the middle of the home defence before firing low past the helpless Mandanda on 83 minutes. Jordan Ayew pulled one back from the penalty spot at the death, but that could not prevent Arsenal from making it six straight wins in all competitions.
They have set themselves up nicely for the remainder of their Group F campaign, with tough fixtures against Borussia Dortmund and Napoli to come, while outsiders Marseille already face an uphill struggle.
They had welcomed Andre Ayew back into their starting line-up after a domestic suspension, while Giannelli Imbula kept his place in midfield despite suggestions that he might be replaced by Benoit Cheyrou. Imbula, who turned 21 last week, impressed on his Champions League debut as the hosts started well inside a Stade Velodrome open on one side and which still resembles a building site.
Aware of the need to get off to a good start, Elie Baup's side had the better of the first-half chances, and the first genuine fright for Arsenal came when Andre Ayew headed a Dimitri Payet cross just wide in the 21st minute. That was in the midst of Marseille's best spell of pressure in the first period, as Arsenal struggled to contain the lively trio of Mathieu Valbuena, Imbula and Payet.
Per Mertesacker, who was back in the Arsenal line-up after missing Saturday's 3-1 Premier League win at Sunderland, then cleared a low Payet cross from practically underneath his own bar before Andre-Pierre Gignac headed just over from Payet's corner.
Gunners grow into game
In contrast, Arsenal created little of note in the first period, with the exception of a Walcott effort which was saved by Mandanda inside six minutes. However, the match was a far more open affair after the restart.
The unlikely figure of Rod Fanni appeared inside the penalty area within two minutes of the second period beginning to send a volley fizzing just past the far post, before Mandanda denied Kieran Gibbs, who had been the beneficiary of a marvellous backheeled pass by Mesut Ozil as the action swung from end to end. As the hour approached, Marseille captain Mandanda was called into action again, saving from Jack Wilshere after the England midfielder was picked out unmarked by Bacary Sagna's cross.
There was then an almighty let-off for the London club, as Gibbs did brilliantly to clear the ball away from under his own bar and avoid a certain own-goal after Mertesacker had managed to spoon a cross over the helpess Wojciech Szczesny.
Four minutes later, Gibbs played a central part in the crucial opening goal, as his cross from the left sparked confusion in the Marseille area and Walcott took full advantage to score.
Ramsey, who scored the only goal when Arsenal won at the same venue in a Champions League group game two years ago, then made sure of the win before Jordan Ayew, on as a substitute, netted a stoppage-time penalty consolation for OM after Ramsey was adjudged to have fouled Andre Ayew inside the area.