Around 13 and a half minutes in on the video
I would consider it on the harsh side for a straight red, but can see why it was given. Arms was last man, Krishna had the jump on him and was definitely pulled back. He was however 30 yards or so out at the time.
As I say, probably harsh, but understandable. O'Leary also took some time and consulted with Assistant first so was no "snap" decision