Mortensen left ITV in May after a restructure of the broadcaster's research division that led to it becoming part of the marketing director Clare Salmon's remit.
At OPera, Mortensen will co-ordinate research across print, radio, TV, online, cinema and outdoor. His task will be to link research from different media to create a more coherent picture for OPera's agencies and clients.
John Overend, the joint managing director of OPera, said: "The need to investigate the convergence in various media in the next few years should provide our agencies' clients with a real advantage in the marketplace. It is a real coup to get someone of Neil's level and variety of experience."
During his time at ITV, Mortensen introduced work including tvWORKS, the ITV Fame Ratings and the ITV Vision Panel.
OPera handles media negotiations for Omnicom's OMD and PHD agencies.
Earlier this month it launched OPera Digital to handle digital negotiations for PHD, OMD and other Omnicom digital agencies.