Loza caused more problems for Southend's second goal, winning a penalty for his side having been fouled in the area by goalkeeper Ryan Clarke.
Lee Barnard stepped up to take the spot kick and sent the keeper the wrong way with a cool finish that doubled his side's lead.
Southend came close to extending their lead even further when Timlin's half volley was well saved by Clarke, who was then called upon to tip Kevan Hurst's corner over the bar to prevent it flying straight in.
A David Hunt corner at the other end caused more problems shortly afterwards as Constable lost his man, only to send his volley over the bar when unmarked at the back post.
Oxford almost pulled a goal back right on the stroke of half time when Rigg diverted a header past the keeper, but Hurst was in the right place at the right time to clear it off the line.
The visitors threatened again just two minutes after the restart when Hunt's throw was met by Constable, who chested the ball just wide.
Southend responded with a chance of their own moments later as Timlin found Hurst, who fired an effort narrowly over the crossbar.
Hurst was involved again in the 68th minute, sending a curling effort towards goal that the keeper just about managed to tip over the bar.
Clarke was called upon again just a minute later as he pulled off a fine save to deny John Egan's header and keep the deficit at two.
The Oxford keeper was kept busy over the next 10 minutes, producing stops to keep out Ryan Leonard twice and another Egan header.
However, he was finally beaten for a third time on the evening with just four minutes remaining as Loza beat two men before clipping the ball over the keeper to seal all three points for his side.