Laird curled in an inch-perfect opener in the 67th minute before former Chelsea midfielder Smith pounced on a loose ball late on to round off a dominant O's display and rain on the parade of Preston boss Graham Westley in his first game in charge.
Westley made five changes to his first Preston side but it was Orient who dominated the first half - David Mooney having an effort ruled out for offside early on.
Dean Cox then should have done better after Kevin Lisbie's strong run and pull-back before the Orient winger stung the palms of Thorsten Stuckmann seconds before the interval.
The visitors got the goal their performance deserved 20 minutes after the restart when Laird, after being released by Leon McSweeney, curled in his first goal in Orient colours.
The game was put beyond doubt 10 minutes from time when, after the home defence failed to deal with a high ball into the box, Smith popped up to fire home the second.
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