The U's made four changes for the trip to Hereford United in the FA Cup Second Round.
Marc Tierney returned at left back, Kem Izzet and Simon Hackney came into midfield, whilst Steven Gillespie started up front for the first time since March.
Phil Ifil, David Fox, Anthony Wordsworth and Clive Platt were the men to make way as the U's started with an attacking line-up.
The home side were to have the first chance of the afternoon, as Constantine cut in from the wing and got his shot away, but it slashed well wide and eventually went out for a throw.
They then broke the offside trap and as McCallum tried to cross the ball, he miscued his effort but saw it clip the crossbar as it went away to safety.
The Bulls were bright in the early stages, and loanee Mutch laid the ball out to the wing before continuing his run to sidefoot volley the resulting cross well over the bar.
However, the U's had the best chance of the first ten minutes when persistence from Gillespie saw him regain the ball by the byline, cutting inside the full back and drilling in a low shot that Bartlett did very well to turn away from danger.
Two minutes later, the U's front man threaded through a superb ball for Odejayi who powered his way to the edge of the area before firing in a shot that was relatively simple for the keeper to collect.
The U's were starting to create chances and from the second of two quick corners, Odejayi flicked a head at the ball and saw it cleared off the line at the last moment.
Lisbie's pace saw the U's escape forward to set up O'Toole to curl an effort wide of the top of the post, before McCallum cut in to fire a shot into Williams' midriff as the mid-point of the half arrived.
Whilst the home side had spells of possession, it was the U's who were the more incisive as their pace proved vital.
Persistence from Hackney saw him stand a cross up to the far post and this time Gillespie was free, but he could only force his header outside of the side netting with his punched headed effort.
The home side had a rare chance just after the half hour mark as the ball was laid across the edge of the area to the onrushing Jones. He smashed in an effort that may have just dipped in, but Williams tipped the ball over the bar with a fine save.
The teams went in level at half time, and both sides made a change at the break. Hackney made way for the U's, having looked in discomfort at the interval, with Platt coming on up front and Gillespie going to left wing.
The home side started the second half the brighter, but the U's continued to defend manfully when required.
The U's were again breathing a sigh of relief eight minutes after half time, when a cross from the left by Valentine eluded Williams, but again clipped the crossbar and away to safety.
Izzet went into the book for a foul on the left hand side and loanee Mutch took the free kick from a dangerous position. The effort was wasted, but it was certainly the Bulls who had seen more of the ball in the second half.
They had one effort well saved just after the hour mark, and the U's fans were urging their side on to try and get a foothold in the game and find a path to goal themselves.
McAllum managed to squeeze his way through the back line with a clever through ball and, jinking back and forth, he eventually struck a shot that went straight down the throat of Williams.
The U's made a change with fifteen minutes remaining as Wordsworth came on for Gillespie in the dying moments of the game, and the visitors slowly but surely stemmed the flow of Hereford pressure.
Indeed, going into the dying stages of the game, it was the U's who were on top and the more likely to score.
And so it proved in the final moments of the match in a well crafted Colchester move.
The ball was cleverly fed out to Wordsworth after some neat build up play by the U's and, although the cross initially went behind the awaiting players, the chance wasn't over yet.
O'Toole strode confidently onto the loose ball and smacked home a great low effort to send the visiting fans into raptures.
The U's players all joined the travelling fans in celebrating the Irishman's goal and the U's made late progress into Round Three.
Att: 2,225 (271 away fans)
U's: Williams, White, Reid, Okuonghae, Tierney, Lisbie, Izzet, O'Toole, Hackney (Platt), Gillespie (Wordsworth), Odejayi (Ifil)
Subs: Cousins, Heath, Batth, Thomas