The 26-year-old, who is a full Scotland international, has struggled for game time with the Millers following his summer move from Sheffield Wednesday.
He joins up with U's boss Michael Appleton for a second time following a spell together at Portsmouth and he could feature in Saturday's trip to Hartlepool.
"I've worked with him before and know he will fit in here," Appleton told the club's official website.
"He is a full international, he has played at a higher level, and he has a fantastic winning mentality. He will want to impress here and can play in a number of roles across the front line.
"We are waiting for confirmation on Ryan Taylor but it looks like he will be out of action for a month or so with the groin injury he picked up on Tuesday against Newport, but we have acted quickly to bring Chris in to the squad and he will be an excellent addition."