A 91st minute strike from Chris Atkinson settled a tight game as Oxford United left Chesterfield empty-handed.
Johnny Mullins had headed United into a first-minute lead and the visitors controlled the opening half hour.
But Chesterfield levelled deep into stoppage time at the end of the first half through Liam Cooper.
Both sides went for the victory after the break, but the U's conceded again in time added on, when Atkinson's shot found the corner.
It was rough on the visitors, who deserved at least a point.
United were forced to make three changes, with Tom Craddock (rib) and Liam Davis (hamstring) unavailable, while Sean Rigg could only make the bench after suffering with flu.
Alfie Potter came in to play up front and there were also recalls for Tony Capaldi and Jake Forster-Caskey.
The U's came flying out of the traps and opened the scoring after exactly 60 seconds.
Mullins rose highest to meet Peter Leven's corner and superbly steered his header into the top corner.
The visitors followed it up with a superb opening 20 minutes.
Composed in possession and carrying a cutting edge up front, their grip on the game irritated the home fans.
It took a magnificent save from Tommy Lee to keep the score down on seven minutes. The Spireites goalkeeper dived full stretch to keep out Forster-Caskey's header from Lee Cox's cross.
It took Chesterfield 24 minutes to muster a meaningful effort on goal, when Tendayi Darikwa's shot narrowly missed the top corner from 20 yards.
James Constable blazed a decent opening over after half an hour, but slowly the home side grew into the game, as Danny Whitaker began to pull the strings.
U's goalkeeper Ryan Clarke had to make a smart save to keep out a low shot from Atkinson, before the first half finished in controversial fashion.
First, United had two penalty shouts waved away in three minutes.
Constable was bundled over by Nathan Smith and with the next attack Leven was convinced his header was blocked by Atkinson's hand.
Chesterfield then drew level when Cooper lashed in a shot following a scramble at a corner.
But United were furious, as the goal came in the fourth minute of stoppage time, twice as long as the fourth official had indicated.
The visitors were on the back foot at the start of the second half.
Atkinson came within a whisker of putting Chesterfield in front on the hour, cutting in from the left and fizzing a shot just past the post.
U's boss Chris Wilder responded by making a bold double change, bringing on Jon-Paul Pittman and Rigg for Simon Heslop and Forster-Caskey.
In turn, Chesterfield brought on experienced strikers Marc Richards and Jack Lester.
The tempo increased as the time began to run out, with both sides doing everything to win the game.
Both United substitutes were quickly involved.
Rigg unlocked the hosts' defence with one jinking run, but his low cross evaded three yellow shirts in the box.
Pittman had sight of goal shortly afterwards, but instead of shooting first time he attempted to control a high ball and the chance vanished.
At the other end Nathan Smith shaved Clarke's crossbar with a rising shot five minutes from time.
The game looked to be heading for a stalemate, but United were again stung in stoppage time.
Atkinson provided the breakthrough, bursting into the box and sliding a shot past Clarke.
Oxford Utd: Clarke, Whing, Mullins, Wright, Capaldi, Heslop (Pittman 61), Cox, Leven, Forster-Caskey (Rigg 61), Potter (Chapman 90), Constable.
Unused subs: Brown, Raynes, Marsh, O'Brien.
Chesterfield: Lee, Talbot, Hird, Cooper, Smith, Atkinson, Togwell, Darikwa, Whitaker (Boa Morte 82), Dickenson (Lester 74), Boden (Richards 67).
Unused subs: O'Donnell, Clay, Forbes, Ridehalgh.
Referee: Dean Whitestone.
Attendance: 5,433 (426 visitors).