During his 22-year stint as the chief of the agency trade body, he was credited with drafting the first UK advertising codes and being a key player in the establishment of commercial TV in 1955.
He set up the Audit Bureau of Circulations, the European Association of Advertising Agencies and was awarded a papal knighthood for masterminding publicity for the Pope's visit to Britain in 1982.
"Of all the IPA director-generals, James was the most impressive," Philip Circus, the IPA's former legal affairs director and an O'Connor hiring, said.
O'Connor, a one-time RAF officer, joined the IPA as a secretary in 1953, becoming director-general two years later. He is remembered for his efforts to establish advertising codes across Europe and in developing areas.
An expert in advertising law, he was often called on to act as an arbitrator in disputes and was a lecturer, broadcaster and author on advertising matters.
He is survived by Joan, his wife of 60 years, and their son and daughter.
His funeral will be held next Wednesday at 2.30pm at St John's Church, Tadworth, near Epsom, and anyone who knew him is welcome to attend.