In the Greek-Italian War (1940/41) Oikonomou was an interpreter of Italian and German in the staff of the 3rd Army Corps. In 1941–45, during the Greek occupation, a small ship served as a residence for him and his wife Eugenia (née Deligianni). They travelled the islands of the northern Aegean Sea and the Gulf of Euboea – until they were suspected and harassed in interrogations by Italians, Germans and even by their own compatriots. His excellent language skills, his sincerity and fearlessness helped Oikonomou to escape the many dangerous situations of that time.