[8] Despite his initial strong performances on the pitch, in March 2015, Marios Oikonomou went through a difficult spell in which he was forced to stay in the bench after the arrival of Daniele Gastaldello.

The transition to the three-man defence allowed him to return to the field and to unlock the game against Livorno by scoring a goal giving three key points in the race for promotion.

But even with the arrival of Gastaldello, Oikonomou has seen it as a stimulus: "I tried to look on the bright side - admitted the defender - from players like him and Domenico Maietta I can learn a lot, they both have a valuable experience.

[9] On 9 June 2015, Marios Oikonomou contributed to Bologna's return to Serie A one-year after their relegation thanks to a dramatic 1–1 draw in the play-offs against Pescara Calcio.

[10] On 20 September 2015, he helped his club to claim their first win since gaining promotion back to Serie A by beating Frosinone 1–0 at the Stadio Renato Dall'Ara.

Bologna took the lead in the 27th minute as Anthony Mounier was picked out by Marios Oikonomou's ball over the top, before lashing a fierce drive across the keeper and into the far corner.

On 28 June 2018, AEK officially announced the signing of central defender Marios Oikonomou, on loan from Bologna until the end of 2018–19 season.

[19] At the end of the first half of the season, according to rumours AEK is ready to offer an amount in the range of €700,000 with the Italian club in order to make his transfer permanent.

[20] On 22 January 2019, AEK consequently decided to pay Bologna the transfer fee in order to sign Oikonomou on a permanent basis, with the player penning a contract with the club until the summer 2022.

[22] A week later, on 6 April 2019, Oikonomou equalizing the score as he prodded home after the ball fell kindly to him in a goalmouth scramble after Petros Mantalos had a shot blocked in a final 3–1 away win game against Xanthi, a result which means the team have all but locked up third place in the SuperLeague.

[28] After he spent several months as a free agent, on 17 February 2023 Oikonomou joined Serie A club Sampdoria on a short-term deal until the end of the season.

On 16 March 2016 Oikonomou was called for the first time to play for the senior Greece national football team in two friendly matches against Iceland and Montenegro.