Deacon joined his first band, The Opposition, in 1965 at the age of 14. The band played covers of chart hits; Deacon played rhythm guitar using an instrument he had bought with money borrowed from the group's founder, Richard Young. He switched to bass the following year after the original bassist was fired for not improving his playing as much as the other members. As well as a dedicated musician, Deacon also was the band's archivist, taking clippings from newspapers of even the advertisements featuring The Opposition. After being in the band for four years, not long after the group cut an acetate of three songs, Deacon played his final concert with the band (then called The Art) in August 1969. He left as he had been accepted to study at Chelsea College in London (now part of King's College London), where he obtained a First Class Honours degree in Electronics in 1971. Having become a fan of Deep Purple, he saw the group perform the "Concerto for Group and Orchestra" with the Royal Philharmonic Orchestra at the Royal Albert Hall that September.