A first-half goal from Hassan Mohamed in the 24th minute and strikes from Karim Adel (44) and Hani Ragab (73) sealed Olympic's crucial win, while Mohamed Ramadan hit Petrojet's only goal of the game on 25 minutes.
The win lifts Mohamed Helmi's men to 15th place in the table with 16 points, as Petrojet remain second on 34.
The hosts started the game brightly and their superiority could have been transformed into an early lead when they won a penalty-kick in the 14th minute.
However, Ibrahim Abdul-Gawad did well to save Mostafa Mosheer's effort from the spot.
Olympic managed to take the lead ten minutes later thanks to Mohamed's header from close range.
It didn’t take long for Petrojet to respond as Ramadan capitalized on a poor clearance from the defence with a low shot in the Olympic net.
Olympic restored their lead one minute from the interval, when Adel did well to meet a right-sided cross with a tap in from close range.
In the second half Petrojet were all out in the attack which exposed their back line.
Ragab took full advantage to put the result beyond doubt for his team, by rounding off a counter attack with a lovely finish past Abdul-Gawad.
While the home players were out of the pitch celebrating Ragab's strike, Petrojet substitute Alaa Ibrahim tried to loop a long effort but his goal was disallowed.
In another game, Ghazl Mahalla came back from a goal down to beat Tersana 2-1 in Cairo.
Amr Samaka scored his first goal since returning to Tersana in the 22nd minute before goals from Mozambique striker Danilho Daniel and Mohamed Salah gave Mahalla the three points.
Khaled Eid's team are now sixth in the table with 23 points, while Tersana remain bottom on 13 points.