James Constable put Oxford in front in the 24th minute with his 96th goal for the club.
Rochdale had lost all three previous away games this season 1-0 but they threatened when George Donnelly shook off three opponents before firing in a curling shot from outside the area that Ryan Clarke saved at full stretch to his right.
And five minutes into the second half, Keith Hill's men were level. Rhys Bennett hung up a cross from the right by-line and Matt Lund headed powerfully home at the far post.
Bennett himself then headed wide with another far-post opportunity.
The U's brought on teenager Tyrone Marsh for his debut, and he went close near the end with a shot that Lillis just managed to block.