The Sunderland star has already spent two months on loan with The Mariners this season, but initially wanted to try and force his way into The Black Cats first-team after being recalled.
However, Oster has now decided to return to Blundell Park where he has played 10 times and scored five goals for the club this season.
Meanwhile, the First Divsion side have announced the capture of Richard Hughes on loan from high-flying Portsmouth.
Mariners boss Paul Groves praised both players' determination to join the club saying to the club's official website: "You have to take your hat off to these two lads who have the desire to come here and play first team football.
"Obviously it was a frustrating time for us, missing out on several of our targets but you have to keep plugging away.
"Others have not taken that opportunity but these players obviously have a great mental strength and a determination which will be a great addition to the squad."
Meanwhile, Groves has hinted the club could move for Ipswich Town's Jim Magilton this summer and that he is also keen on bringing in a new striker.
"We haven't made an enquiry for Magilton, but he's a good player and I wouldn't rule anything out at the moment," added Groves.
"We've been looking for a midfielder, but our attentions have switched to a centre forward."
