The distance to second-tier salvation could grow wider later in the day if West Bromwich Albion beat Preston.
Meanwhile, Derby’s victory moved John Eustace’s team to within two points of Hull City in the play-off spots above them, although that gap could be widened if the sixth-placed Tigers beat Birmingham City later in the day.
Oxford were left exposed for the 22nd-minute opener after a long throw into Derby’s box was cleared out for Banel to latch onto.
The Burnley loanee raced away without hesitation and, despite having team-mates peel off into space around him as Oxford’s defence scrambled back, the Ajax academy product went it alone to score his second goal in as many home matches.
It was a moment of individual brilliance and the only shot on target in a tense first half that produced few other chances.
It was not until after the break that Derby goalkeeper Zetterstrom was called into action, with the Sweden international helping keep Peart-Harris out.
Zetterstrom rushed off his line to ensure Oxford were foiled, but it was the determined backtracking run of Szmodics and the forward’s perfectly timed tackle on Peart-Harris that crucially denied Oxford a leveller.
Szmodics goal-saving intervention also came just moments after he threatened at the other end.
With a little over 20 minutes remaining at Pride Park, a rare defensive error from Matt Clarke gifted Konak the ball but, despite having plenty of space and options to exploit, the Brentford loanee squandered the gift.
Szmodics almost snatched a second for Derby from a long throw, while Zetterstrom had to thrust a boot out to keep a heavily deflected pass from Aidomo Emakhu out before the substitute teed up Mark Harris for a last-minute chance that was sent wide by the U’s forward.