The Super League statement described the situation as "a critical moment" and explained the move was about their "responsibility to limit the spread of Covid-19".
Olympiakos topped the table at the end of the regular season with 66 points, 14 ahead of second-placed title holders PAOK Thessaloniki.
The two clubs are set to be joined by AEK Athens, Panathinaikos, OFI Crete and Aris Thessaloniki in a round-robin tournament to decide the league champion.
A playoff was also scheduled for the bottom sides in the league to determine which club would be relegated.
"Each of us, every citizen of Greece, must fight his own battle so that we can all come together to emerge victorious from this ordeal, the pandemic that afflicts the world," the league said.