The U's have now gone five games without a win as their automatic promotion hopes continue to fade.
Jamie Cureton poached an equaliser in the 84th minute as he deflected substitute Zack Kotwica's shot over goalkeeper Ryan Clarke.
Oxford had taken the lead in the 78th minute through Ryan Williams' second goal in successive games, the Australian winger drilling in a low shot from 15 yards to break the stalemate.
That came just seconds after Cheltenham had squandered a great chance when Cureton's glancing header at a free-kick was diverted on to a post by Clarke's outstretched foot.
Oxford top scorer James Constable had earlier threatened the most in a game which saw defences on top.
Constable headed just wide from David Connolly's right-wing cross and also sent a fierce 25-yard drive over the crossbar.
A superb burst from Constable in his own half should have brought a goal but, from his pass, Deane Smalley's touch let him down and Robins keeper Scott Brown, quickly off his line, made a smothering save.
Town's ex-Oxford midfielder Sam Deering saw a long-range dipping shot saved by Clarke before the break.