Oikonomou was involved in a collision between his motorbike and a vehicle in Ioannina on 23 May. He sustained severe head injuries and was taken to hospital, where he underwent several emergency operations.
According to reports in Italy, the former centre-half underwent a decompressive craniectomy, a procedure in which part of the skull is temporarily removed to relieve pressure on the brain. He was later admitted to intensive care but did not recover.
Members of the Greek football federation expressed their condolences to Oikonomou's family and friends.
“The president of the federation, Maquis Gkagkatsis, executive secretary Domna Tsioni and the members of the Executive Committee of the Federation express their deep sadness and sincere condolences to the family and loved ones of Marios Oikonomou, who left this life prematurely at just 33 years old,” a statement said.
Oikonomou had been without a club since 2024, following a spell at Panetolikos. During his career, he also played for Giannina, Cagliari, Bologna, SPAL, Bari, AEK, Copenhagen and Sampdoria.
While with Copenhagen, he won the Danish title in 2022. He won six caps for Greece, the most recent coming in 2018.