Dagenham kept their lead over second-place Oxford to 11 points thanks to substitute Sam Sloma's late goal.
Sloma drilled the ball home in the 90th minute after Oxford failed to clear.
Earlier striker Paul Benson had grabbed his 25th league goal of the season with a close-range finish to give the visitors a deserved first-half lead.
But Yemi Odubade's 70th-minute shot made it 1-1 and soon afterwards he pounced on keeper Tony Roberts' error to lob the ball into the empty net.
Oxford Utd: Turley, Luke Foster, Quinn, Gilchrist, Anaclet, Pettefer (Robinson 67), Johnson, Rose (Hargreaves 46), Odubade, Zebroski (Duffy 67), Burgess.
Subs Not Used: Tardif, Day.
Goals: Odubade 70, 72.
Dag & Red: Roberts, Foster , Uddin, Arber, Griffiths, Rainford (Vernazza 87), Leberl, Southam (Saunders 78), Moore (Sloma 62), Benson, Strevens.
Subs Not Used: Eyre, Cole.
Booked: Griffiths, Leberl, Uddin.
Goals: Benson 17, Sloma 90.
Ref: Gavin Ward (England).