Ollie Banks gave Paul Cook's side the lead inside ten minutes before Gary Roberts doubled Chesterfield's advantage from the penalty spot just eight minutes later.
Two red cards and an injury meant the Us played out the final ten minutes with just eight players and Roberts rounded off the scoring in stoppage time to seal three points for the Spireites.
It was a bright but blustery day in Derbyshire and Chesterfield were nearly hit by a tornado within two minutes of the outset when a freekick was only half cleared to Oxford’s Dave Hunt on the edge of the penalty area. He caught the ball truly with a first time volley, and the goalscoring would have been opened but for keeper Tommy Lee athletically tipping over for a corner.
Awoken from their slumber, the Spireites went almost as close on six minutes when Roberts won a free-kick 23-yards out and took the kick himself, just grazing the top of the netting with United keeper Ryan Clarke at full stretch.
Hardly had the blue-shirted fans regained their seats however, and Chesterfield were in front. Jay O’Shea’s cross from the right alluded the Oxford central defenders, was chested down by Banks at the back post and blasted low and home for his second goal of the week.
After Oxford’s up-tempo start and Chesterfield’s goal-laden response, the hosts were keen to get their foot on the ball and consolidate their lead, and were soon given an almighty helping hand by the visiting defence. A swinging set piece caused danger, but not nearly as much as the penalty needlessly conceded inside the box for a reckless push.
Chesterfield were in no mood to be so charitable, and stormed into an early two-goal lead when Gary Roberts smashed home the resulting spot kick.
Approaching the half hour and the visitors were looking to push forwards and get themselves back in the game, though Chesterfield were rarely looking as though they were going to lose their composure.
The home side nearly finished the first half with a flourish, when some sublime approach play from Banks and Marc Richards found an opening for O’Shea who would have put the cherry on top of a memorable half were it not for a last gasp defensive challenge.
Far from resting on the laurels, a handball within 30 seconds of the restart gave the Spireites the chance of a third goal with a free-kick 25-yards out. O’Shea sized up the opportunity, got his kick up and over the wall, but not quite down in time to trouble Clarke in the away goal.
Despite hardly having created an opening a golden one came the way of Oxford towards the hour as winger Ryan Williams burst past despairing challenges to find himself through on goal. However, lacking composure, the player on loan from Fulham could only blast a low shot at the legs of the onrushing keeper when he should have given his side a foothold in the game.
For the first time since the opening knockings, Chesterfield were on the back foot. Another chance soon fell the way of Johnny Mullins after Lee had only parried a corner. To the relief of the home fans, the 'keeper was able to recover quickly enough to bravely block the incoming shot and retain his side’s two-goal advantage.
For a spell the bit seemed firmly between United teeth, but the Chesterfield confidence was clear to see and by the time new loan signing Mason Bennett came on for his debut with twenty minutes to go the ship had been steadied.
The hosts were indeed the next to go close when Roberts’ set piece from wide right found the head of Ian Evatt six yards out. The effort had the captain’s authority but was inches too high and thumped against the top of the crossbar.
Within a minute the Oxford challenge became all the more difficult as they were reduced to ten men. Banks received the ball in the middle of the park only for visiting left back Tom Newey, who had earlier given away the penalty, to slide straight through the back of him. The crowd gasped and referee Paul Tierney showed little hesitation in producing a straight red card.
Even the ten men of United didn’t last for long, as a further three minutes later they were reduced to nine. Michael Raynes was already on a yellow card and his rough challenge twenty-five yards from his own goal was enough for the centre back to earn a second yellow and an early departure.
As the clock ticked over into the three minutes of added on time the visitors lost yet another player, Johnny Mullins leaving the field on a stretcher and leaving only eight yellow shirts to see out the closing minutes.
With the very last kick of the match Chesterfield did finally grab their third, with a wonderfully worked goal. Eoin Doyle played a slide-rule ball through to Roberts, who was able to sprint past the tired defence and lift the coolest of finishes over the 'keeper and cap off a fine afternoon at the Proact
With the League 2 chasing pack also winning the picture remains unchanged at the top of the table, but ahead of the long trip to Plymouth on Tuesday Chesterfield again showed they sit atop league fully on merit.
Oxford United: Clarke, Newey, Raynes, Wright (c), Smalley (Constable 70'), Potter (Connolly 78'), Hunt, Williams, Rose, Mullins, Ruffels (Rigg 78').
Subs not used: Crocombe, Whing, Wroe Bevans.