The borough has also been nominated as the region's representative for Britain in Bloom 2016 in the Best City category.
The judges awarded the borough the top prize, along with a 'Gold' rating, at an awards ceremony held in Southport. This is now the sixth year in a row the borough's entry, Bloom and Grow, has travelled back home with the trophy. Oldham also bagged the Best Town Centre in the North West award.
This year's theme showed how the town is "pulling together and driving change".
The WOW bed centrepiece display in the town centre includes a red phone box and a Hackney cab, designed by council leader Jim McMahon and his two sons. The display contains images of the town centre's past, present and future, and currently sports a giant furry spider to mark Halloween. It will be transformed further for Christmas.
This year's judging route also took in Alexandra Park, the Boot and Beanstalk display on Emmott Way, Gallery Oldham, Huddersfield Road, including Waterhead War Memorial and Waterhead growing hub, Stoneleigh Park and the secret garden.
Cllr McMahon said: "Bloom and Grow is a team effort so to win Best City for the sixth year in a row is a truly outstanding achievement and a real honour for the borough. As in the past our entry has put a smile on the faces of members of the public and also the RHS judges.
"This award reflects the hard work, dedication and commitment from all those residents, schools, communities and businesses that were involved."