The pair hold off competition from Train, who climb up two places to second with 'Hey, Soul Sister'.
Country pop trio Lady Antebellum shoot up 15 spots to third with 'Need You Now', followed by Lady GaGa and Beyoncé's 'Telephone' and David Guetta and Kid Cudi's 'Memories' in fourth and fifth respectively.
The highest new entry of the week comes from Mika and Red One, who land in eighth spot with 'Kick-Ass'.
Outside of the top ten, the Glee cast have a new entry at 14 with their cover of Bonnie Tyler's 'Total Eclipse Of The Heart'.
The top ten singles in full (click for our reviews):
1. (1) Usher ft. will.i.am: 'OMG'
2. (4) Train: 'Hey, Soul Sister'
3. (18) Lady Antebellum: 'Need You Now'
4. (3) Lady GaGa ft. Beyoncé: 'Telephone'
5. (5) David Guetta ft. Kid Cudi: 'Memories'
6. (8) Diana Vickers: 'Once'
7. (6) Timbaland ft. Justin Timberlake: 'Carry Out'
8. (-) Mika ft. Red One: 'Kick-Ass'
9. (7) Scouting For Girls: 'This Ain't A Love Song'
10. (11) Taio Cruz ft. Ke$ha: 'Dirty Picture'