His hit 'OMG' - also a recent chart-topper in the UK and US - notches up a third week at number one.
B.o.B.'s collaboration with Paramore singer Hayley Williams, 'Airplanes', leaps from seven to two, while Perry's 'California Gurls' is the highest new entry of the week, debuting at three.
Meanwhile, Ke$ha climbs from 13 to eight with 'Your Love Is My Drug' to notch up her third Aussie top ten hit in a row.
Notable gains outside the top ten come from Sean Kingston and Justin Bieber, who jump from 21 to eleven with 'Eenie Meenie', and Rihanna, who advances from 35 to 28 with 'Te Amo'.
The top ten singles in full (click where possible for our reviews):
1. (1) Usher ft. will.i.am: 'OMG'
2. (7) B.o.B. ft. Hayley Williams: 'Airplanes'
3. (-) Katy Perry ft. Snoop Dogg: 'California Gurls'
4. (2) Taio Cruz: 'Break Your Heart'
5. (17) Yolanda Be Cool & DCup: 'We Speak No Americano'
6. (5) B.o.B. ft. Bruno Mars: 'Nothin' On You'
7. (3) Brian McFadden ft. Kevin Rudolf: 'Just Say So'
8. (13) Ke$ha: 'Your Love Is My Drug'
9. (6) Jason Derulo: 'Ridin' Solo'
10. (8) Train: 'Hey, Soul Sister'