Powerful second-half headers from Marc-Antoine Fortune and substitute Theo Robinson gave Southend United a 2-0 win at in-form Oxford to boost their League One play-off hopes.Oxford had won their previous five games in all competitions but they struggled to break through against a dogged Shrimpers defence.Just after the hour mark - after a pacy overlap on the right - Jason Demetriou delivered a perfect cross for Fortune to head powerfully home.And with just two minutes remaining, Robinson smartly peeled away from the goal-line to head in Luke O'Neill's right-wing corner.Oxford were not at their best and their better opportunities came in the first half.Chris Maguire fired an angled 20-yard free-kick into the side netting.And from a Maguire corner, centre-half Chey Dunkley was unlucky to see his thumping header come back off the inside of the post.Dunkley had another header saved at a corner after the break, but Southend were always threatening in the second half and sealed maximum points.