Rendell ran onto Peter Leven's pass over the top to open the scoring with a cool low finish in stoppage time at the end of the first half.
And Rotherham were two down seven minutes after the break when Hall fired high into the net from Andrew Whing's cutback.
Ben Pringle and Sam Hoskins missed good opportunities for the visitors before they were given a lifeline with an 83rd-minute penalty.
Adam Chapman tripped Jason Taylor just inside the box and Lewis Grabban buried the spot-kick into the bottom left corner.
Michael Duberry twice headed off the line from Danny Harrison in the closing minutes, while Oxford also went close when Dean Morgan's looping header was nodded off the line by Michael Raynes.
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