Plymouth had taken the lead through Maksym Talovierov in the 38th as City looked in danger of another embarrassing defeat in a seasonÂ
O Reilly the Academy graduate score two headers, one in each half, ensured Maksym Talovyerov’s opener would not see the Championship strugglers to their third Premier League scalp of the season.
Captain Kevin De Bruyne rounded off the win by tapping home the third as the game entered stoppage time.
O’Reilly’s performance marked his second and third goals in senior football for City after opening his account against Salford City in January.
The success means City are into the quarter finals of the historic competition for the seventh successive season.
City will discover there opponents in the last eight when the draw is made on Sunday evening.