The Daggers have got to do it all again in a replay after battling to a draw with a vastly improved performance, writes Jack Steggles.
A goal from Danny Green cancelled one from Alex Revell to save it for the Daggers at that was the least they deserved.
In view of their poor away record cynics will already be writing off their chances of making further progress when they travel up the road to give it another go. But they have got to get that elusive first win on their travels at some stage - so why not at the home of their near neighbours
If they put in the same effort as they did in this full blooded affair they will certainly have nothing to fear. They gave every bit as good as they got to give boss John Still encouragement for the future.
Still shuffled the pack, left out Josh Scott and Gavin Tomlin and gave wholehearted Jon Nurse a start up front alongside Bas Savage. There was also a debut for Victor Palsson, on loan from Liverpool. And the Icelandic Under-21 international did enough to suggest he will be a useful asset.
There was also a place in the starting line up for keeper Chris Lewington. And he wasted no time making his mark, saving well from Alex Revell after only two minutes.
Revell provided the O's main attacking threat and skipper Mark Arber headed another effort from him out from under the bar in the 15th minute.
Orient, beaten 2-0 in a League One game at Victoria Road earlier in the season were clearly determined there would be no repeat and fought ferociously for every ball.
Lewington had to race from his goal and hack the ball clear when a poor back pass from Solomon Taiwo dropped the Daggers in the cart.
Bit John Still's men slowly got the bit between their teeth, worked up a head of steam and put Orient on the back foot. Keeper Lee Butcher held a header from Romain Vincelot, Savage put one wide and Green flashed a shot across the face of the goal.
McCrory fired over following good work by Savage, who was holding the ball up well and making a nuisance of himself. And it took a brilliant block by Terrell Forbes to rob Nurse as he got set to pull the trigger.
Chances had come and gone at both ends before Revell finally made one count, firing in a left footer in a swift Orient break on the stroke of half time.
The lead last just 30 seconds before Green had the Daggers back on terms. He fired in a free kick from the edge of the area that hit Butcher's right hand post, bounced off the keeper himself and went in.
The pace never slackened after the break as the two local rivals slugged it out. Dean Cox fired a good effort just past the far post and Lewington did superbly to save from the same player with his legs when a goal looked certain.
At the other end Arber just failed to get his head on a deep cross from Green and Vincelot was too high with a rasping 20 yarder.
The result was about right and Still had plenty of reasons to be cheerful afterwards. He claims his team are slowly getting better and promises to add two more players to the squad to make the second half of the season more comfortable.