However, neither of the top two teams was particularly convincing against the bottom two - a composed Jacob Bergstrom finish with 25 minutes remaining was all Mjallby needed to overcome Oster 1-0 away in a match filled with missed opportunities at both ends.
That was sufficient to keep the Hallevik side a point ahead of Hammarby, who required a last-minute goal to secure a 1-0 victory of their own against last-placed Varnamo, who had Rufai Mohammed sent off after 19 minutes.
The home team finally capitalized on their numerical advantage in the 91st minute, when Moise Kabore rose highest to head in an Abdelrahman Boudah cross.
While 1-0 victories were the theme for the top two, teams in third and fourth faced 2-0 defeats.
AIK were stunned on the road by Halmstad, who pulled themselves clear of the relegation zone with two goals in a nine-minute span during the first half.
Pawel Chrupalla opened the scoring on 20 minutes, before Naeem Mohammed added a second to secure Halmstad a second win in their last three matches, condemning AIK to their second loss in the same number of games.
Also enjoying a 2-0 triumph were Hacken, marking their first win in four matches, as they defeated Elfsborg. Two goals from John Dembe were decisive - the first coming after 33 minutes, followed by a second in stoppage time of the second half.
Elfsborg is now without a win in three matches and three points behind AIK, but remain in fourth place due to a 0-0 draw between the teams in fifth and sixth - Malmo and GAIS.
This marks a second consecutive draw for the champions, although the previous one was against Mjallby, while GAIS continues their strong form; they remain unbeaten in their last seven matches.
Unexpectedly, goals were plentiful for Djurgarden, who have only netted 14 goals in 14 games this season, including the five they scored in a 5-1 thrashing of Degerfors.
August Priske seemed to be the star of the match with two goals in the first 24 minutes, yet he was outshone by Tokmac Nguen, who scored twice, once before halftime and again from the penalty spot to complete his hat trick, just two minutes from the end.
Marcus Rafferty briefly gave Degerfors hope early in the second half to make it 3-1, but that proved to be a mere consolation as they fell to their fifth consecutive defeat, while Djurgarden remains in eighth place.
The Iron Stoves were unable to advance as Goteborg defeated Sirius, 3-1, despite going behind to an early Leo Walta penalty.
Max Fenger equalized with a penalty of his own before halftime, then he completed the comeback three minutes from time, with Sebastian Clemmensen capping the victory in added time.
On Monday night, a Daleho Irandust goal in the 88th minute was sufficient for Brommapojkarna to claim a 1-0 away victory against Norrkoping, moving BP up to 10th place, above their opponents.
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