Grenoble win play-off to bounce back to Top 14
Oyonnax, who were promoted last season and then finished 13th in the top division, made a disastrous start to the game and were outplayed in every area by the home team who scored seven ries.
Grenoble had missed their first opportunity to win promotion when they lost the D2 final to Perpignan last weekend, but they grabbed the second chance.
Fullback Lolagi Visinia touched down for Grenoble after eight minutes and scrum-half David Mele scored a second try three minutes later.
Oyonnax hit back when wing Daniel Ikpefan scored the first of his two tries.
But Grenoble responded as props Ali Oz and Sona Taumalolo both touched down and led 28-10 at half-time.
After Ikpefan crossed again in the 51st minute for the "Oyomen", winger Bastien Guillemin, Visinia and replacement hooker Paulin Mas scored for the home team in a 14-minute burst.
Grenoble were relegated last year after a nightmarish season during which six of their players were embroiled in a rape investigation.
The club started this season with the biggest payroll in D2, finished third in the table but then edged runners-up Montauban in the championship semi-final to ensure either direct promotion or a play-off against Oyonnax.