Substitute Ryan Leonard scored four minutes after coming on as a substitute to earn Southend a late victory which takes them up to fifth in the table.
Leonard headed Michael Timlin's cross past new Oxford keeper Luke McCormick, who was playing his first Football League game since being jailed in 2008.
McCormick, signed from Truro City on deadline day, was barracked by the crowd but had little to do.
His opposite number Paul Smith twice denied Alfie Potter in the first half.
Lineup,
Southend United
01 Smith
02 Clohessy Booked
04 Spillane
06 Cresswell
15 Phillips
23 Barker
08 Timlin Booked
14 Hurst (Leonard - 81' )
26 Laird (Reeves - 81' )
10 Corr Booked
20 Assombalonga (Lavery - 67' )
Substitutes
17 Bentley
03 Straker
18 Leonard
27 Reeves
16 Prosser
07 Eastwood
13 Lavery
Oxford United
24 McCormick
02 Batt
04 Raynes
06 Wright
23 O'Brien (Smalley - 88' )
07 Chapman
14 Rigg (Davis - 67' Booked )
20 Leven
09 Constable
15 Potter
18 Richards
Substitutes
31 Crocombe
17 Capaldi
22 Worley
08 Heslop
25 Parker
26 Davis
10 Smalley
BlueWes
65
tannis
62
McVicar
59
nstratty
56
Dirk P
54
longbottomd
54
awjoufc
51
Johnners8
6
sean66oufc
0
StillaSpursFan
0
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