Millwall 4 Barnsley 1: Heckingbottom left to rue Cup exit
After the visitors had gone ahead early on, the game turned during a 20-minute spell either side of half time, as O’Brien scored two goals and created another while Joe Williams saw red to remove any hope of a comeback.
Fred Onyedinma added a fourth to make sure as Millwill ended a run of five straight defeats against Barnsley in emphatic fashion.
Barnsley were the better side in the first half an hour, and took the lead in the 11th minute, when Adam Hammill’s floated cross sailed over the head of Tom Bradshaw, but fell kindly to Brad Potts to lash home.
They might have doubled their lead through Bradshaw or Dimitri Cavare before their hosts grew into the game, but both were off target with their efforts.
Jed Wallace switched wings from right to left and Millwall really began to threaten. He provided a hint of what was to follow in the 27th minute, when he floated in a cross which Onyedinma could not direct at goal.
In the next seven minutes he would create chances for O’Brien and Conor McLaughlin, before finally seeing the former convert at the second time of asking.
The cross that produced the equaliser might have been meant for Onyedinma, but when he failed to control it, the onrushing O’Brien made no mistake, firing low and hard through Adam Davies and into the corner.
The two Millwall wingers almost combined for a second before the break, as O’Brien turned provider and Wallace poked his cross past Davies but just wide of the far post.
Just 91 seconds into the second half the home side were ahead. O’Brien did well to get to the byline and cut back for Ben Thompson, who finished emphatically to make it 2-1.
Barnsley were falling apart, and things went from bad to worse five minutes later when Williams was shown a straight red for a vicious, late challenge on Wallace.
Millwall, and O’Brien in particular, took full advantage, the winger controlling a speculative Wallace shot brilliantly before curling in a his second from the edge of the box.
Steve Morison then produced a delicate flick to play in Onyedinma, who chipped the ball over Davies to make it four after an hour.
Morison and Jake Cooper both had chances to extend the lead, but Millwall largely seemed happy with what they had, and Barnsley rarely threatened a second goal.
Barnsley boss Paul Heckingbottom was left to rue his side’s defending as he had no complaints about the defeat.
He said: “Although it was 1-1 at half time I felt we’d missed an opportunity and I saw signs that we weren’t really defending properly.
“The second goal was a clear indication of that and then the sending off obviously puts us on the back foot.
“I’m really disappointed. However, the only saving grace is that there’s not three points lost on this game today.”