Luka Milivojevic scored Olympiakos's second goal with a direct freekick.
Olympiakos won the top-of-the-table clash with second-placed Xanthi on Sunday to stretch its lead at the Super League table to 12 points, while AEK and PAOK are in trouble after fresh losses. Panionios has now joined Xanthi in second.
The Reds braved the cold at Xanthi and won 2-0 thanks to goals by Brown Ideye and Luka Milivojevic, while both teams finished the game with 10 men each.
Panionios triumphed 2-0 at Panetolikos with goals from Masoud Shojaei and Giorgos Masouras to climb to 25 points from 14 games, the same as Xanthi. Panathinaikos can join them if it beats visiting Kerkyra on Monday.
AEK lies sixth, 16 points from the top, after going down 3-2 at Asteras Tripolis on Saturday, with its coach, Jose Morais being close to the exit. AEK led via Tomas Pekhart, Asteras advanced 2-1 through Aleksandar Stanisavljevic and Igor Silva, Pekhart equalized for AEK before Stavros Tsoukalas gave the hosts victory 10 minutes from time.
PAOK is even lower, in eighth, suffering a shock 1-0 home loss to fifth-placed PAS Giannina due to an early Themistoklis Tzimopoulos goal.
In other games Atromitos defeated Levadiakos 2-0 and Platanias saw off Larissa 3-2.
The match between the bottom two teams, Veria and Iraklis, was postponed due to a frozen pitch at Veria.