The controversial deadline day signing was making his first Football League appearance since being jailed in 2008 for causing the death of two young children by dangerous driving.
He served half of a four year and seven month sentence and was handed a hostile reception on his return to the professional game by the Roots Hall crowd.
But he was hardly troubled between the sticks until Shrimpers substitute Ryan Leonard headed home an 85th minute cross from Michael Timlin.
Before then, Oxford wasted the two best chances of an uneventful game with Alfie Potter failing to convert two clear cut chances to score.
The first chance came after just 30 seconds when he was denied in a one-on-one situation by the legs of Shrimpers goalkeeper Paul Smith.
And Smith then did well to keep out a similar effort from Potter in the 39th minute, tipping his low 12-yard shot wide of the left post.