Sheffield United booked an FA Cup fourth-round clash at home to Charlton after coming from behind to see off a spirited Orient side.
The League One strugglers had deservedly gone in ahead at the break after JJ Melligan held his nerve to score from the penalty spot at Brisbane Road.
But this re-arranged tie swung the Blades way after the break as three goals in a 10-minute spell from Greg Halford, Billy Sharp and Kyle Naughton knocked the wind out of the hosts' sails.
Halford added his second of the game with 12 minutes left.
l Millwall set up a fourth-round tie with either Newcastle or Hull as they beat Crewe 3-2.
Crewe took the lead through Luke Murphy but were immediately pegged back by a Scott Barron strike before Neil Harris restored the visitors' lead.
Shaun Miller broke free in the box to put Crewe level before Zak Whitbread's winner.