Substitute Tom Craddock scored twice as Oxford beat Southend to secure their second successive League Two win.
The visitors played for more than an hour with 10 men after Michael Timlin picked up two yellow cards.
Craddock, who missed most of last season through injury, came on at half-time and turned in Alfie Potter's cross to open Oxford's account.
The forward struck again late on as he hit an unstoppable drive past stranded keeper Cameron Belford.
VIEWS FROM THE DRESSING ROOM
Oxford United manager Chris Wilder told BBC Radio Oxford:
"It was another decent night's work for us.
"We've seen these types of night before, but we were really patient tonight.
"We kept the ball well, moved it from side to side. Tommy [Craddock] coming on gave us another goal threat and it was a very satisfying result.
"They made it hard, their manager is an astute man, and the only disappointment is that we didn't round it off with a few more goals."
Southend manager Paul Sturrock told BBC Essex:
"In the first 25 minutes, we were starting to settle ourselves down, we were getting around their goal and beginning to steal the ball in the right areas.
"The second yellow card (resulting in Timlin's dismissal) was a soft one and it changed the whole complexion of the game.
"I'm proud of every one of my players, the work ethic was different class when we were down to 10 men.
"We lost a bad goal 10 minutes into the second half but it was late on before they scored again and everybody gave it their best shift."
Oxford Utd 01 Clarke 02 Batt 04 Raynes 06 Wright 17 Capaldi 07 Chapman 14 Rigg 18 Forster-Caskey 19 Cox (Craddock - 46' ) 10 Smalley 15 Potter (Evans - 89' ) Substitutes 21 Brown 03 McGinty 23 Evans 08 Heslop 09 Constable 11 Pittman 29 Craddock Southend 34 Belford 02 Clohessy 03 Straker (Barker - 77' ) 06 Cresswell 15 Phillips 08 Timlin Dismissed after an earlier booking 14 Hurst (Hall - 65' ) 18 Leonard (Assombalonga - 65' ) 19 Spicer 12 Ferdinand 21 Tomlin Substitutes 01 Smith 16 Prosser 23 Barker 04 Hall 20 Assombalonga 09 Harris 24 Benyon