The striker gave the Bees the advantage in the 43rd minute with a clever back header which looped over goalkeeper Jamie Jones and in off the post.
Jamie Scowcroft went closest for the visitors midway through the first half but fired a half-volley straight at keeper Wojciech Szczesny.
Orient came into the game after the break and only the Bees keeper denied Adam Chambers who was clean through on goal.
But Brentford were always a threat, Grabban again going close when his looping drive was tipped over by Jones just after the hour mark.
Sean Thornton flashed an angled drive wide for Orient before Charlie MacDonald had a shot tipped wide at the other end. Szczesny was the Brentford hero in injury time when he dived late to tip a low goal-bound effort from Sean Thornton wide of the upright.