The Gunners started the afternoon promisingly but failed to make a breakthrough in the first half, with City coming out for the second period with renewed energy.
Pep Guardiola's side dominated the ball throughout the second half and capitalised on a mistake from Kepa Arrizabalaga to take the lead.
The Spaniard let a harmless cross slip through his palms and go into the path of O'Reilly to head home - with the defender popping up again in a similar fashion just four minutes later to double his side's advantage.
Arsenal finished the match strongly but ultimately were unable to break down a solid opposition defence, extending their wait for another trophy under Mikel Arteta.
For Guardiola, it is his 19th piece of silverware while in charge of Manchester City, adding another medal to his illustrious near-decade-long career in England.
More to follow...