After obtaining his degree, in 1958 he went to Paris to pursue doctoral studies under Paul Lemerle.
The outcome of his work on Escorial Taktikon and the other Taktika (seating lists in Byzantine imperial banquets) was published in 1972, as the Les listes de préséance byzantines des IXe et Xe siècle, containing a translation and commentary on the Taktika.
Oikonomides returned to Greece, but the establishment of the dictatorial Regime of the Colonels in 1967 forced him to go to exile in Canada along with his wife, the Ottomanist Elizabeth Zachariadou.
In July 1969, he accepted the chair of Byzantine history at Montreal University, a post he kept until 1989, when he returned to Athens.
His work there led to the creation of a new scientific journal, the Studies in Byzantine Sigillography, which he edited.