The teams were locked in a 1-1 stalemate in the first leg at the Seeb Sports Stadium on Thursday. Senegalese player Ibrahim Kamara was the hero of the unfancied club as he struck a brace that included the decisive goal late into injury-time.
Kamara had put the hosts ahead in the 22nd minute, capitalising on a defensive lapse from the Mussanah players. However, Mussanah equalised six minutes later through Oman international Mohammed al Ghassani, whose free-kick from the top of the box beat the Jaalan wall.
The match looked set to go into extra-time with the aggregate score on 2-2, but Kamara blasted a powerful left-footed volley in the 93rd minute to spark celebrations among the home supporters.
Jaalan, coached by Tunisian Mohammed al Adhari, had finished third in the first-division league behind winner Rustaq and Oman clubs to earn a promotion playoff with Mussanah.
Batinah side, Mussanah, had ended 12th in the 14-team OPL with 29 points and had to survive against Jaalan to stay in the top flight, but its loss relegated it for the first time since the start of the OPL in 2013.
Sur and Salalah were the other teams relegated for the next season.