O'Shea began work with the giant squid whilst working for the National Institute of Water and Atmospheric Research (NIWA) in New Zealand in 1996. He became the Director of the Earth and Oceanic Sciences Research Institute at the Auckland University of Technology in 2005, and was a Discovery Channel Quest Scholar. In 2009 he resigned from his Discovery Quest position to focus research on coastal conservation, environmental matters and postgraduate supervision and teaching; despite this he has remained active in squid research, but is best known (in the popular press) for his involvement with large cephalopodtaxa, particularly Giant and Colossal Squid, and their preservation. In 2011 he resigned from his position at the Auckland University of Technology.
O'Shea has published extensively on cephalopods, fisheries, whale diet and shallow subtidal coastal ecology. He presently resides in Bordeaux as a photographer.