Karl Duguid delivered a low cross from the right in the 28th minute which Forbes, under pressure from Ian Henderson, turned past his own goalkeeper Lee Butcher from four yards out.
The visitors missed a golden opportunity to increase their advantage a minute after the break when Anthony Wordsworth split the Orient defence to send Gavin Massey through with only Butcher to beat, but after rounding the goalkeeper the striker put his shot wide from 10 yards.
Orient's best chance arrived after only seven minutes, but Kevin Lisbie shot straight at goalkeeper Ben Williams from just outside the six yards box.
Lisbie seemed to be brought down by Magnus Okounghae just before the interval, but the referee waved away appeals for a penalty.
Orient piled on the pressure in their search for an equaliser in the last 20 minutes, but were unable to get the better of a resolute Colchester defence, which provided a fine protective shield around Williams.
Any reproduction, publication or redistribution of this material without the written agreement of 365 Media Group is strictly forbidden.