In 1999 Stewart was asked to produce a song on an album by Primus, an American alternative rock band whose bassist, Les Claypool, was widely acknowledged to be one of the most interesting musicians of the decade.

Stewart was so impressed by Les that the two decided to jam together on some improvised music. Les had been invited to play a festival in New Orleans in 2000, and he asked Stewart and guitarist Trey Anastasio of Phish to join him for the performance. The three of them rehearsed many classic tunes, including "House Of The Rising Sun" and various Led Zeppelin tracks, while also writing some original material.

That 2000 concert marked an unforgettable night for many. Critical reception to the performance was enthusiastic, and the band's jam sessions had produced a number of good songs. Although it took many months for them to play together again, Stewart, Les, and Trey reunited to record an album under the name of Oysterhead.

The band released the album The Grand Pecking Order in October 2001, and a tour of the U.S. saw them playing in front of screaming crowds, all waiting to hear these three incredible musicians on the same stage together. Once again Stewart found himself working as a drummer in a rock band, and his performance was so impressive that many expressed disappointment that a band like the Police could not reform and demonstrate once more the great potential still held by three such legendary musicians as Andy Summers, Sting, and Stewart Copeland. None of them had ever said officially that the Police had broken up, so many fans still held out hope; but it seemed that none of them was making any effort to bring the band back together either.