A 1-0 win over Halmstad on Saturday had seen Mjallby's lead grow to 11 points and heap pressure on Hammarby, but they responded in style at home to struggling Oster.
The hosts were in front three minutes thanks to Oster giving the ball away far too casually inside their own half, allowing Paulos Abraham to race away and score.
The second goal arrived in all too simple fashion midway through the half, as Nahir Besara's corner was headed in by Victor Eriksson. Another Besara set piece made it 3-0 shortly before the break, this time he produced a wonderful free-kick which flew into the top corner.
That wrapped up the points for the Bajen, who added a final flourish four minutes from time when Sebastian Tounetki buried a penalty into the bottom corner. Hammarby still have a mountain to climb to catch Mjallby, while successive defeats keeps Oster in the relegation play-off place.
Back-to-back wins - the latest a 2-1 victory over Sirius - have allowed AIK to move back into third place, but sitting five points shy of Hammarby, a late title bid looks unlikely for the Solna side.
They had to do it the hard way, as late in the first half, Robbie Ure's strike hit the post and bounced off the back of diving goalkeeper Kristoffer Nordfeldt and in for an own goal.
The early stages of the second half featured a hatful of chances at either end, but just before the hour mark, it was AIK who finally took one, when an Anton Saletros cross was swung in from the left and met beautifully by Erik Flataker.
Three minutes later, the turnaround was complete - Abdihakim Ali took aim from 25 yards and his strike took a deflection, giving goalkeeper David Celic no chance. It was a cruel way to end Sirius' four-match unbeaten run, as the Uppsala side stay in 12th place.
AIK have replaced Malmo in the top three, after the title holders were held to a surprise 1-1 draw at home by struggling Degerfors.
The home fans at Eleda Stadion were stunned in the 10th minute, as Malmo afforded Marcus Rafferty time and space to take aim from the edge of the area and pick out the bottom corner.
MFF got their act together late in the half and scored a fine equaliser when Hugo Bolin slotted a nice ball through to Adrian Skogmar, who finished well on the turn despite a tight angle to work with.
In the second half, Robin Olsen was the hosts' hero with a penalty save to deny Arman Taranis just after the hour mark, and was called upon late on to preserve a point. It's a second draw in succession for Malmo, while Degerfors pick up just their third point in their last 13 league outings, staying 15th.
Malmo are now only hanging onto fourth place on goal difference as GAIS beat Hacken 2-1 in a Gothenburg derby to move ahead of another side from the city, IFK Goteborg.
The Mackerels took the lead inside the first 20 minutes - an accurate Matteo de Brienne cross from the left was met by Amin Boudri, who guided his header in off the post.
The advantage was doubled with 18 minutes to play when William Milovanovic left the Hacken defence in his wake and rolled the ball across to Gustav Lundgren, and though the goalkeeper kept out his initial effort, Lundgren followed in well and tucked away the rebound.
A Julius Lindberg effort from the edge of the area reduced the deficit for the UEFA Conference League qualifiers, but it wasn't enough to stop them falling to a fourth league loss in seven since they began their European fixtures.
Finally, another win for Djurgarden has kept them in the running for European qualification next season, as they brushed aside 10-man Norrkoping, 4-0 away from home, to go seven league matches unbeaten and only six points off the top three.
Two goals in nine minutes from August Priske - one header and one left-foot strike from close range - had already set Djurgarden on their way before two bookings in the space of three minutes saw Yahya Kalley dismissed, reducing the home side to 10 men.
A sweet Oskar Fallenius strike midway through the second half put the result beyond doubt, before an equally-impressive hit from Nino Zugelj added the final flourish for the Iron Stoves, who stay eighth. Norrkoping sit 11th after their two-match winning run was soundly ended.