Live-wire striker Nicky Ajose struck the only goal as npower League One newcomers Crawley continued their promising start to the season with a third win in four league games.
Ajose, on loan from Peterborough, scored in the 72nd minute with a well-placed, low shot after being set up by substitute Michael Jones.
Orient, who have now lost their opening three league matches, shaded the first half and Crawley goalkeeper Paul Jones was forced to parry a goal-bound shot from debutant Martin Rowlands in the first minute.
Moses Odubajo then shot just wide of Jones's left-hand post following a penetrating run as the visitors enjoyed plenty of possession.
Skipper Gary Alexander shot wide on the turn for Crawley and Hope Akpan was narrowly off target from a David Hunt cross shortly before the break.
Orient keeper Ryan Allsop saved well to deny Nicky Adams after Billy Clarke's neat pass played in the Crawley winger.
Visiting striker Michael Symes, set up by Gary Sawyer, should have hit the target after 58 minutes when he shot wastefully wide from a good position.
But Orient also had a let off when Ajose rounded Allsop only to roll the ball wide.