Posh really turned on the style in the second half, scoring four goals to blow their hosts away.
Gabriel Zakuani gave them the lead just before the break, stooping to head home a right-wing cross from Gavin McCann.
Jabo Ibehre then levelled for the U's, smashing home after being teed up by strike partner Clinton Morrison following a neat move involving Alex Gilbey and Andy Bond.
But Posh proved far too strong in a one-sided second half.
Tyrone Barnett's header from Danny Kearns' cross put them back in front, before Tommy Rowe hammered an unstoppable effort past U's goalkeeper Sam Walker, after collecting the ball from Lee Tomlin.
Tomlin himself then took centre stage, curling an exquisite effort into the top right-hand corner before drilling home after making space for himself on the edge of the box.
Posh had also gone close in the first half when McCann's curling free-kick was brilliantly tipped over by Walker.