Video: Sheffield United need to cut out individual errors to gain promotion, says Blades midfielder Stefan Scougall
Scougall cancelled out Shaun Williams' opener with a deflected shot that caught-out Millwall goalkeeper Jordan Archer, before the game's turning point in the 89th-minute.
O'Connell, who appeared to suggest he was pushed, handled the ball in the United box and after referee Andy Woolmer pointed to the spot, Steve Morison made no mistake to keep Chris Wilder still searching for his first win as Blades boss.
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Hide AdAnd Scougall said: "We're disappointed, obviously. We think we did more than enough to deserve a point and towards the end, there was a mistake. Jack's held his hands up in the changing room and we'll take it on the chin. It's a cruel game but that's football sometimes."