Grimsby had produced a defiant performance but looked ready to settle for a valuable point.
But in the first minute of injury time Steve Livingstone burst down the left and swung in a cross to Oster who smashed in his shot from ten yards.
And Grimsby fans were quick to cheer off Georges Santos after he produced an impressive one-man show.
Ronnie Moore's Rotherham were sick of the sight of the towering French defender as he ran the match.
But the miserable Millers were left still looking for the elusive 50-point target Moore set as the amount required to keep first division football at Millmoor for another season.
But the night belonged to Grimsby and ex-Sheffield United favourite Santos who was head and shoulders above the rest and kept Rotherham's three-man strikeforce at bay.
He patrolled the back with masterful ease and defied Alan Lee, Richard Barker and Paul Warne to beat him. They failed miserably.
Defender John McDermott joined in the heroics heading off the line when Barker finally found a way past Santos to nod Lee's crafty lob goalwards.
Santos even found time to turn up in attack alongside lone frontman Darren Mansaram and produced a rare threat hammering a 20-yard shot that drifted wide.
Grimsby had two anxious moments in as many minutes early in the second half.
First John Mullin's free kick was headed back across goal and Lee's looping header dropped against the upright and was pawed away by Danny Coyne.
Seconds later Gareth Farrelly, on loan from Bolton, looked set to crown his home debut with a winner. He picked his spot but Tony Gallimore appeared from nowhere and cleared off the line.
But at the death Oster came up trumps to give Grimsby their second win in four days.