The essential difference between the Premiership and First Division sides was the Israeli midfield player Eyal Berkovitch, who was a significant and delightful influence throughout. He evaded Oxford's attempts to persuade him out of the contest with some brutal tackling and twice came back from crunching challenges. His other misfortune was that he had to pass to Southampton players who frequently wasted what he offered them.
This had happened twice - Gordon Watson was most culpable after a lovely diagonal through ball - when Berkovitch took matters into his own hands. As he received Watson's square pass from the left in the 22nd minute, through the legs of defender Matt Elliott, he artfully drew the goalkeeper and easily did the rest.
Oxford could hardly manage a touch of the ball in midfield for much of the rest of the first half and resorted to trying to touch Berkovitch around his ankles instead. They might have been two down before the break - Berkovitch's shot being interrupted by a last-ditch tackle - but instead found themselves equalising.
Les Robinson's long cross from the right flank looked to have more speculation than intent about it but it dropped perfectly for the stooping Nigel Jemson, who glided in his 16th goal of the season past a static Dave Beasant.
This seemed only a minor set-back for Southampton 13 minutes into the second half. First, Jim Magilton was fortunate to get in a cross after putting his free-kick against the wall and Richard Dryden out-jumped his markers. Egil Ostenstad ran half the length of the field a minute later, rounded the goalkeeper and hit a post before Berkovitch intervened again. In his side's next attacking move he rounded two players, drifted into the area only to see his shot parried. Ostenstad pounced on the rebound which should have been that, except that from the restart Bob Ford was allowed to rampage unhindered upfield. He reached the edge of the area, defenders to a man stood off and the shot curved past Beasant, who was still resolutely immobile.
However, it was Beasant after a frenetic finale who produced the save of the night, low down to his left from Mike Ford two minutes from time to ensure Southampton's progress.
Southampton (5-3-2): Beasant; Van Gobbel, Dryden, Madison, Lundekvam, Oakley; Magilton, Berkovitch, Slater; Watson, Ostenstad. Substitutes not used: Potter, Hughes, Monkou.
Oxford United (4-4-2): Whitehead; Robinson, Elliott, Purse, M Ford; Beauchamp, Smith, Gray (Massey, 81), B Ford; Jemson, Aldridge (Moody, 64). Substitute not used: Rush.
Referee: P Durkin (Portland).