With six club battling it out for two direct spots in the top-flight league from the first division, Rustaq's win helped it total 19 points and take an unassailable lead.
Oman Club is second with 15 points and Jalan is third with 14, and will fight it out for the other promotion berth.
The third-placed team in the first division league will face the 12th-placed OPL team in a playoff for a spot in the top flight. In the last round on April 22, Jalan hosts Rustaq while Oman will host Seeb and Bausher meets Nizwa.
Earlier, these six teams had qualified for the second stage from a group of 14 teams in the first division.
Three teams from each group of seven made it into the final stage.
Last week, Salalah was relegated from the OPL, while Sur, Al Shabab, Al Mussanah and Muscat are hovering in the relegation zone, with two rounds to go.
Salalah, however, is in contention for a berth in the prestigious His Majesty's Cup final.
It will play away in Sohar where it faces hosts Al Khabourah on Sunday in the return-leg semifinal after playing out a goalless draw at home.
The other second-leg semifinal, also on Sunday, pits hosts Al Nasr and Saham at the Al Saada Sports Complex. The first leg in Sohar had ended in a goalless affair.
Meanwhile, the penultimate OPL round will be held on Wednesday with all the 14 teams in action. Fanja is on top of the standings with 45, with Suwaiq trailing only on goal difference.
Defending champion Al Arouba is in third place with 44 points.
The concluding round will be held on April 24.