The Brewers striker was given space to turn and fire in a low shot from 16 yards with 20 minutes remaining and cancelled out James Constable's opener in the League Two clash at the Kassam Stadium.
Oxford gave a debut to 19-year-old goalkeeper Max Crocombe, who did well in the testing, blustery conditions.
Burton keeper Stuart Tomlinson produced a great reaction save after 33 minutes when Damian Batt had got round the visitors' defence to meet Adam Chapman's cross on the volley.
But Oxford went ahead when Constable marked his return from a four-match suspension by heading home.
The striker peeled away from the near post to nod in Tony Capaldi's corner five minutes into the second half for his 10th goal of the campaign.
But Maghoma levelled for Burton in the 70th minute with a nice finish for his side's first goal in their last four away games after a start to the year in which they had previously won all their games at home, and lost all of them away.
Billy Kee had the chance to grab a late winner for Burton, but sliced horribly wide.