The pair beat off competition from the Glee cast, who take second place with their cover of Madonna's 'Like A Prayer'.
Lady GaGa and Beyoncé' move up two spots to third with 'Telephone', while Train's 'Hey, Soul Sister' and David Guetta and Kid Cudi's 'Memories' claim the remaining positions in the top five.
James O'Connor and Audrey Trainor's 'Falling Up' is the highest new entry of the week, landing in ninth position.
The top ten singles in full (click where possible for our reviews):
1. (2) Usher ft. will.i.am: 'OMG'
2. (19) Glee Cast ft. Jonathan Groff: 'Like A Prayer'
3. (5) Lady GaGa ft. Beyoncé: 'Telephone'
4. (4) Train: 'Hey, Soul Sister'
5. (6) David Guetta ft. Kid Cudi: 'Memories'
6. (8) Timbaland ft. Justin Timberlake: 'Carry Out'
7. (7) Scouting For Girls: 'This Ain't A Love Song'
8. (3) Diana Vickers: 'Once'
9. (-) James O'Connor & Audrey Trainor: 'Falling Up'
10. (1) Glee Cast: 'Gives You Hell'