After a short postdoctoral appointment at the Carnegie Institute in Pittsburgh, Olive returned to Cambridge as a Fellow of Churchill College, becoming a Lecturer in the Department of Applied Mathematics and Theoretical Physics in 1965. Here he made key contributions to the approach to particle physics known as S-matrix theory. His 1966 book The Analytic S-matrix co-authored with Richard Eden, Peter Landshoff and John Polkinghorne, remains a definitive text on the subject and is known as ELOP. In 1971, Olive made what he has described as a "momentous personal decision" to sacrifice his tenured position in Cambridge and move to the Theory Division, CERN as a fixed-term staff member. He was part of a team assembled by :de:Daniele Amati|Daniele Amati to work on the theory originally known as the dual resonance model but shortly to be recognised as string theory. In CERN, Olive began the collaborations with the circle of string theorists many of whom feature in his memoir From Dual Fermion to Superstring. His work at CERN, in part in collaboration with Lars Brink and Ed Corrigan, initially focused on the consistent formulation of dual fermion amplitudes, generalising the existing bosonic models. This period saw several of Olive's major contributions to string theory, including the Gliozzi-Scherk-Olive projection which elucidated the role of spacetime supersymmetry in ensuring consistency of the dual fermion model and was to prove an essential step in establishing 10-dimensional superstring theory. He was one of the first to become convinced of the conceptual revolution whereby string theory is viewed as a unified theory of all particle interactions, including gravity, rather than simply as a model of hadrons. This was the subject of his plenary talk at the 1974 Rochester conference in London. In 1977, Olive returned to the UK to take up a lectureship at Imperial College, becoming Professor in 1984 and Head of the Theoretical Physics Group in 1988. He had by now begun collaboration with Peter Goddard and together they produced a series of papers on the mathematical foundations of string theory, notably on Virasoro and Kac-Moody algebras and their representations and relation to vertex operators. One outcome of their work on algebras and lattices was the identification of the special role played by the two Lie groups SO and E8 x E8, which would shortly be shown by Michael Green and John Schwarz to exhibit anomaly cancellation that led to the renaissance of string theory in 1984. This body of work from 1973 to 1983 was recognised with the award of the prestigious Dirac Medal in 1997 to Goddard and Olive "in recognition of their far-sighted and highly influential contributions to theoretical physics. They have contributed many crucial insights that shaped our emerging understanding of string theory and have also had a far-reaching impact on our understanding of 4-dimensional field theory.” The Dirac Medal also recognised a second major line of research pioneered by Olive, on duality symmetries in gauge field theories, this work was to play a key role in later developments of M-theory. While still at CERN, Olive had begun to study the magnetic monopoles which 't Hooft and Polyakov had shown existed in non-abelian gauge theories, publishing a paper with Peter Goddard and Jean Nuyts. In 1977, together with Claus Montonen, he made the remarkable conjecture that there should exist an electromagnetic dual theory in which the roles of monopoles and gauge bosons are interchanged. In subsequent work with Ed Witten, Olive showed that this duality is realised in a certain class of supersymmetric theories. This Montonen-Olive duality was later found to emerge from a deeper web of dualities underlying M-theory, ushering in the of the mid 1990s. In 1992, Olive left Imperial to take up a research professorship in mathematics and physics at Swansea University, where together with Ian Halliday he built the theoretical particle physics group. He continued to work on mathematical physics, exploring the deep symmetries underlying quantum field theories, especially affine Toda theory. His retirement was marked by a conference "Strings, Gauge Fields and Duality" held in his honour in Swansea in 2004. He presented the Dirac Lecture at DAMTP on 14 June 2004 titled The Eternal Magnetic Monopole.
