Leeds were on top at Portman Road before O’Kane clashed with Knudsen in an off-the-ball incident.
Referee Robert Jones was close to the clash and instantly branded a red card for O’Kane.
And the sending off sparked a strong reaction from Leeds fans.
Writing on Twitter, one supporter said: “Eunan O’Kane what are you doing you stupid t*** just getting on top of them and you do that.”
Another raged: “For f*** sake just when we look on top Eunan you f****** idiot it goes from bad to worse.”
While a third said: “Eunan O’Kane gets away with murder for his performances. He’s been terrible.”
O’Kane was in the news last year for seeking a humanist marriage to model Laura Lacole and some fans used that as inspiration for their comments.
One said: “I knew Eunan O’Kane was trouble when he decided not to get married in the home of our lord and saviour Jesus Christ.”
While another joked: “Eunan O’Kane must of been provoked. He’s only human(ist) after all.”
There was limited support for O’Kane but one fan said: “I get that O’Kane has made a huge mistake but he’s been a rock all season bar one or two games so I don’t get why all the hate is still flowing in.
“A lot of us were screaming for him to be captain early on…”