Jed Wallace opened the scoring in the 20th minute, David Connolly made it two six minutes later and, after Andy Wright had been sent off, Patrick Agyemang wrapped up a 3-0 victory six minutes before the break.
Southend have made their best start to a season in 23 years and share top billing with Oxford.
The Shrimpers defeated Northampton 2-0 at Roots Hall today thanks to Anthony Straker (38) and Freddy Eastwood (62).
Oxford lead the table on goal difference after a 3-1 victory over Torquay.Striker James Constable was instrumental in Chris Wilder's side claiming their third league win in a row, scoring a superb 62nd-minute goal and laying on the third for fellow substitute Asa Hall (78).
More top news
Philippa has the latest weather for the east
Philippa has the latest weather for the west
Nazanin Zaghari-Ratcliffe's five-year jail sentence is confirmed despite her husband's campaign to bring her home.