Phil Brown's men came out on top in Monday's clash between two promotion rivals at Roots Hall thanks to a blistering start, orchestrated by Loza, that included two goals in the opening 17 minutes.
The on-loan Norwich City forward first supplied the cross that eventually saw Will Atkinson double his league tally for the season before winning the penalty that Lee Barnard converted after being upended by Oxford goalkeeper Ryan Clarke.
And Loza made it a night to remember with an assured finish from the edge of the penalty area four minutes from time after beating two defenders.
Oxford had earlier been thwarted by a string of fine Daniel Bentley saves as Brown's side made a timely return to winning ways by securing a first victory since the end of January.
The win hauls Southend, who now occupy the final play-off place at the expense of York City, three points behind Oxford, with Gary Waddock's first game in charge ending in bitter disappointment.
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