Hirst created Wednesday's first. His 12th-minute run into the Saints' box was illegally curtailed by Ken Monkou, John Sheridan converting the spot-kick. Hirst doubled the visitors' lead from 30 yards in the 63rd minute before Monkou made slight amends with a late header.
Crystal Palace also triumphed by the narrowest of margins, Simon Osborn's second-half volley earning them victory at Middlesbrough and a sixth successive success. Forget fears of falling into the Football League: Palace, on current form, are steaming towards Europe.
No goals and, for 17 minutes, no floodlights at Selhurst Park, where Wimbledon, Palace's troubled tenants, had the better chances but failed to extinguish Chelsea.
Newcastle, the First Division's leading light, fell to their heaviest defeat of the season - 4-2 at the Manor Ground. Oxford went two up in the first 10 minutes, through John Durnin and Nick Cusack, before Liam O'Brien halved the deficit with a delightful 20-yarder. Oxford then forged ahead when Steve Howey was judged to have brought down Chris Allen, Jim Magilton stroking home the penalty. Lee Clark kept the Geordies in the chase but Magilton's gem of a free- kick in the 82nd minute ended any lingering hopes.
Newcastle remain 12 points clear as second-placed Tranmere slumped 3-1 at Barnsley, where John Aldridge missed a penalty. The capital challengers moved within range of Rovers, West Ham drawing 2-2 at home to Luton (after throwing away a 2-0 lead) while Millwall beat Leicester City 2-0 at The Den. Portsmouth leapfrogged over Leicester into fifth by overcoming Derby 4-2 at the Baseball Ground, Guy Whittingham recording his 28th and 29th goals of the season.