The visitors, already aggrieved by the award of the penalty, then found their task even more difficult with the dismissal of defender Joe Oastler on the stroke of half time.
Mathew Stevens came into the side to make his home debut for the Rebels, and he had an impactful debut.
City were first to threaten as a header flashed wide from a corner, before Stevens had his first effort, coming inside from the right before having a thunderous shot blocked.
The game’s first controversial moment came in the 15th minute. Chris Flood played a lovely reverse pass for Stevens to run on to, and as he bore down on Craig King’s goal, he went to ground as the keeper made contact with both the ball and the Slough striker. Referee James Durkin pointed to the spot, and after numerous Oxford City protests Dobson stepped up to confidently tuck the spot kick away against his former club.
City had a good chance to level when Reece Fleet played a through pass to Matt Paterson, who could only put the ball wide from the edge of the box as Jack Turner rushed out of his goal.
Stevens was again involved as he crossed in for Flood, who couldn’t quite turn the ball goalwards as it bounced up around his midriff.
In the final minute of the half, Dobson intercepted a City attack and launched the ball downfield for Stevens to chase. The Slough striker was in a foot race with Oastler who, as last man, was adjudged to have pulled him to the ground. James Durkin was again quick to make his decision and Oastler was shown a straight red card.
Slough were then very fortunate to benefit from an offside flag early in the second half as City broke away with Tshimanga clearly in his own half when the ball was played to him.
Warren Harris headed a deep cross from Dobson wide and George Wells saw a low drive held by King as Slough looked for a second goal.
And when the goal did arrive, it was handed to them by the unfortunate Fleet, who turned a corner kick from Dobson into his own net under pressure from Flood in the six yard box.
Slough almost netted a third after a good team move. Lee Togwell played in Stevens, who moved into the box but bent his shot wide.
On 69 minutes, City almost got back into the game. A deep cross from the right was met on the volley by Zac McEachran, and whilst the ball got past Turner – who had spread himself to try and block it – it needed to be cleared off the line by a stretching Lee Togwell.
Paterson fired another warning shot wide and then on 74 minutes Fleet lifted the ball over the top for Paterson, who was denied by Turner. The ten men of Oxford City were not afraid to keep playing passing football in an attempt to turn the game around.
But as they did so, gaps opened up at the back and Stevens almost capitalised on a mix-up between King and Fleet. There was confusion as Stevens claimed he was fouled on the edge of the box, but play continued and the Slough striker fired off-target.
Ben Harris had the ball in the net in the final minute but from an offside position as he touched in Wells’ cross.
Stevens had one final chance deep into stoppage time as he found himself clean through on goal again, but this time King tackled him cleanly.
Oxford City Line up
Craig King, Kyran Wiltshire, Ben Jefford, Josh Ashby, Luke Ruddick, Joe Oastler (c), Zac McEachran, Reece Fleet, Matt Paterson, Carl Stewart, Kabongo Tshimanga. Substitutes: Steven Caro, Craig Fasanmade, Arti Krasniqi, Tom Carter.