A 77th minute Mitchell Mallia strike was all it took for the visitors to claim the lead, despite Liam Youlley receiving a red card in the 85th minute.
The result gives Marconi a crucial three points and will see them break away from a deadlocked race for fifth position and finals football.
It also means Olympic will be facing an uphill battle to finish the regular season within the top five.
It was a hard-fought battle from the get-go at Belmore, with both teams fighting to make an early impact on the game.
Olympic had a chance in the 27th minute as a misdirected clearance landed at the feet of Alex Sanchez in prime position in front of goal, but Nenad Vekic produced a stellar save to keep the game scoreless.
Marconi were denied opportunities in the 57th and 71st minute of the match, but they didn’t have to wait long before a misdirected ball from Billy Patramanis saw Marconi with a throw-in in line with the penalty spot.
A bicycle kick from Tsekenis at the edge of the 18-yard box, landed with Mallia, who fired his shot into the netting.
Disaster struck for Marconi in the 85th minute as Liam Youlley was awarded his second yellow card from Katie Patterson, leaving the visitors with just 10 men in the late stages of the game.
Olympic took advantage of their extra player and mounted pressure on Marconi until the final whistle, but they weren’t able to get the equaliser.
Olympic coach Abbas Saad said the game “could have gone either way”.
“It was just a battle out there,” he said. “They didn’t play much football, we just had to deal with it.
“We played well, we played with a lot of guts and we took it out to them but unfortunately we just couldn’t find the back of the net.”