Southend strengthened their hopes of automatic promotion from League Two while weakening Oxford's chances of a play-off place.
The U's started well and Dean Morgan forced Cameron Belford into a smart save, with Jon-Paul Pittman lively.
But Ryan Hall fired the Shrimpers ahead with a 25-yard free-kick that flew past debutant keeper Connor Ripley.
Bilel Mohsni doubled the lead when he headed Peter Gilbert's cross into the bottom corner.
Southend are in fifth place, but lie just a point behind Crawley in third place heading into the final weekend.
Oxford, meanwhile, are in eighth place, three points behind in-form Crewe.
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Southend United manager Paul Sturrock told BBC Essex:
"The fat lady still hasn't sung yet, so we go into the last game of the season with something to play for."
"I'm delighted with the performance. The work-rate and attitude of the players was unbelievable.
"Right through the second half, it was going to back to the walls, but the boys worked very hard. They put in a shift.
"Defensively we were very sound today another clean sheet, which we're delighted about."