Languishing in the Championship relegation zone, Plymouth had stunned Premier League leaders Liverpool in the fourth round after beating top-flight side Brentford in the third round.

Pep Guardiola's men were in danger of becoming Plymouth's latest famous scalp when Maksym Talovierov gave the visitors the lead in the first half at the Etihad Stadium.

But Manchester-born O'Reilly, making just his sixth start for City, emerged as the unlikely hero with a brace that ensured his troubled team would avoid their lowest moment in a turbulent season.

Kevin De Bruyne was also on target for City, who are hoping FA Cup glory can salvage a woeful campaign that sees them fourth in the Premier League and eliminated from the Champions League and League Cup.

Beaten by Manchester United in last season's FA Cup final, City are through to the quarter-finals as they look to win the competition for the third time in the Guardiola era.

Guardiola has never hidden his love for the FA Cup and this week said: "No other domestic cup competition in world football can compete with its beauty".

The Spaniard still changed his entire starting line-up from Wednesday's league win at Tottenham.

Liverpool had paid the price for fielding a weakened team in their 1-0 loss at Plymouth and City found themselves in peril as well.

City forward Phil Foden's deflected strike trickled just wide before Conor Hazard saved Ilkay Gundogan's header.

- City escape -

Plymouth sat deep in a bid to frustrate City, who were inches away from ending the visitors' resistence when Gundogan fired against the post from 10 yards after Jack Grealish's cross found the German midfielder.

Argyle's plan was working perfectly as Guardiola's team laboured to make headway before falling behind to a 38th minute sucker punch.

Ukraine defender Talovierov rose above the flat-footed City defence to meet a corner with a superb header that gave Stefan Ortega no chance as it arrowed into the far corner.

Vitor Reis thumped a header just wide as shell-shocked City tried to climb off the canvas.

Hazard sprawled to his left to make a fine save from De Bruyne's low drive.

O'Reilly dragged City level on the stroke of half-time when the 19-year-old midfielder met De Bruyne's pin-point free-kick with a glancing header that Hazard got a hand to but couldn't keep out.

Guardiola sent on Erling Haaland in a bid to break the deadlock in the second half and Foden immediately shot just wide.

De Bruyne's curler was pushed away for a corner by Hazard and, from the set-piece, Haaland blasted high from six yards.

Hazard made a brilliant save to keep out Haaland's shot from just two yards, but plucky Plymouth finally succumbed in the 76th minute.

It was a goal tinged with controversy as O'Reilly appeared to get away with a push on Plymouth's Jordan Houghton before burying his header past Hazard from a corner.

In stoppage time, Haaland's shot was saved by Hazard, with De Bruyne on hand to complete City's escape.