Al Suwaiq bounced back from two goals down against unfancied Musannah to punch in four second-half goals and register a 4-2 victory at the Seeb Sports Stadium.
At the same time, Fanja defeated Al Nasr 2-0 at the Sultan Qaboos Sports Complex (SQSC).
Suwaiq and Fanja will take on each other in the decider at the SQSC, starting at 7.40pm.
The title showdown will be preceded by an international friendly between Oman and Iran at the same venue, starting at 5pm.
After Sunday's final round of games, Suwaiq and Fanja jointly topped the 14-team top league with 50 points each.
At Seeb, forwards Daeej al Balushi (17th minute) and Issa al Qusaibi (23rd minute) scored a goal each for Musannah to give the team an unlikely 2-0 lead at half-time.
However, shortly after Musannah keeper Suleiman al Jadidi limped off with injury and the lesser experienced Hani al Rushedi took his place, Suwaiq applied pressure and bounced back in style.
Suwaiq striker Alabd al Nofli fired in his team's first goal in the 49th minute.
Forward Hassan Rabee’a then headed in smartly in the 69th minute, thanks to a cross from Fahd al Jalboobi.
In the 71st minute, Jalboobi himself found the target from a goalmouth melee after the Musannah keeper failed to expertly clear a long shot from Rabee’a.
Suwaiq’s Brazilian player, Anderson, struck his team's fourth goal in the 83rd minute from an impressive solo effort.
Mussabah al Saadi, Suwaiq coach, said after the turnaround win, “The team was nervous in the first half and we didn't defend well at all in that period. However, I am happy with the end result as my boys did come back strongly.”
At the SQSC, Fanja’s foreign player from Senegal, forward Cisse, and its key midfielder Mohammed Mubarak scored a goal each in their team’s victory.
Meanwhile in Sohar, third-placed Saham outplayed Oman Club 4-1 and finished its campaign with 48 points.
Its chances of bagging the title, however, disappeared with Suwaiq and Fanja posting victories on Sunday.
Elsewhere, Dhofar edged out Al Nahda 2-1 while Seeb posted a 1-0 win against Sur.
For Oman Club, the defeat relegated it to the first division. On Saturday, Salalah had become the first elite league side this season to be relegated after losing 0-6 to Al Shabab.
Also on Sunday, Talee’a beat Al Arouba 2-1 and will play with first-division side Sohar to avoid relegation.