The midfielder's measured volley after 53 minutes was the difference between the sides after Aaron McLean had cancelled out Scott McDonald's opener.

It was Posh who began the brighter and Dean Keates should have done better after scuffing a left-footed shot beyond the post and Jamie Day tried his luck from long range.

But it was the Bhoys who opened the scoring in the 19th minute when Dutch striker Jan Vennegoor of Hesselink flicked a free-kick across goal and McDonald made no mistake with the volley.

The lead lasted just two minutes, however, when McLean was on hand to meet Rene Howe's cutback to level things up.

United boss Darren Ferguson rang the changes at half-time, making 10 changes, and it was the visitors who took control, with Vennegoor of Hesselink coming close to restoring the advantage.

And Gordon Strachan's team did not have long to wait to regain the lead courtesy of O'Brien's composed finish.

McDonald was celebrating again 10 minutes later but his goal was ruled out for offside.

The home side did their best to force an equaliser with Shane Blackett close with a header but their efforts proved in vain.