Anthony Wordsworth scored twice as Colchester trounced Oldham for their first home win of the season.
Wordsworth fired them ahead from the edge of the area but Reuben Reid equalised with a penalty.
The U's regained the lead when Wordsworth scored from the spot after Zander Diamond was ruled to have shoved Ian Henderson.
Henderson headed the third from Michail Antonio's cross before Matt Heath nodded in a Lloyd James corner.
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Oldham manager Paul Dickov told BBC Radio Manchester:
"We're constantly on about starting games well and keeping ourselves in the game, and we've conceded after 30 seconds in the first half and 30 seconds in the second half.
"You can't come to places like this and give yourself a mountain to climb, which is what we've done today.
"No goal is conceded by just one mistake, it's usually a combination of errors, and we've seen that with all their goals today."