Another sign of life came in the 44th as Alexander's dipping shot from the edge of box dropped marginally over the crossbar.
In the last minute of the half Alexander's shot on the turn flew dangerously across the face of the goal and there was still time for Lee Steele to spring the offside trap and pull it back for Joe Keith, whose sidefooted effort skimmed the bar as Orient finished the half the stronger.
Eight minutes after half time the East Londoners drew level. From a Joe Keith through ball, Lee Steele pulled free of Charlton right back Luke Young and shot straight at Myhre, the ball just seeping under the Norwegian’s body and over the line.
For a couple of minutes then it was all Orient, the visitors’ momentum whisking their team goalward towards their vocal fans and their 'East, East London' chants. In the brief onslaught, Steele and John Mackie had shots charged down by an anxious Charlton defence.
When danger man Alexander powered a header two yards wide in the 63rd minute, Alan Curbishley had seen enough and made a triple substitution, a radical move with half an hour to play.
The three players replaced – Kishishev, Shaun Bartlett and Brian Hughes were all cheered off with approval by the disgruntled home fans but luckily the gamble paid off as the three attacking replacements Alexi Smertin, Jerome Thomas and Jay Bothroyd all caused Orient problems and turned the match in Charlton’s favour.
While Bothroyd went up front to partner Bent, Thomas weaved his magic on the left wing with a display of feints, stopovers Indian Matka and shuffles that sold a showroom of dummies to the Orient right back Justin Miller while Smertin pitched camp in the hole just outside the Orient box as an irritant who would not go away.
Three minutes after his entry, Thomas proved his worth by turning inside Miller and Daryl McMahon and hitting the side netting. Then he drew a one handed save from Garner in the 72nd after collecting a lay off in the box from fellow sub Bothroyd.
Seconds earlier Bent looked to have won a penalty when hauled down by the backtracking Gabriel Zakuani but referee Alan Wiley ignored the appeals.
Orient had been forced to take second billing again, though roared back in the 77th when Keith picked out McMahon with a diagonal pass that the midfielder headed powerfully, but straight at Myhre.
The closing minutes were dominated by Charlton, Bothroyd, Bent and a deflection from his own player Michael Simpson all testing Garner and Ambrose seeing a goalbound shot blocked by Zakuani with a minute and half left on the clock.
The referee’s assistant had just replaced his board signaling three minutes added on when Charlton won a free kick in a dangerous position twenty yards from goal.
Matt Holland tapped the ball sideways to Bothroyd who let fly with a grasscutter that appeared to take a slight deflection as it bounced off Garner’s prone body and into the roof of the net.
It seemed harsh on Orient to have conceded only seconds from a money-spinning replay at Brisbane Road, but Premiership class, with a helping of lady luck, told in the end.
Post match, Addicks boss Alan Curbishley seemed relieved to have won through to the 5th Round of the Cup although paid tribute to his plucky visitors.