Cooper also shocked fans during his own gig in London last month (Jun11) by inviting Hollywood actor Johnny Depp to join him on guitar.
And now the rocker has made another impromptu performance - by joining Ke$ha onstage for a rendition of his hit track School's Out.
Cooper, who is in the country for his own scheduled gig on Wednesday night (06Jul11), stopped by to see the TiK ToK hitmaker during her sound check, before the pair decided to treat fans during the evening concert.
In a post on Twitter.com after the show, Ke$ha writes, "Thanx for coming to Oslo to do 'SCHOOLS OUT' with me dad... LEGEND. F**KING LEGEND."
Cooper then replied: "Thanks @keshasuxx! Next time our stage, and be sure to bring some bandaids..."
It's not the first time the two stars have appeared onstage together - in October (10) Ke$ha briefly joined Cooper at his Nashville, Tennessee gig.