There were a few nervous faces around Bramall Lane as defender Ben Davies made contact with O'Hare inside the penalty at the beginning of the second half.
However, referee Michael Salisbury waited before awarding a free-kick to the Blades and showing a yellow card to the Coventry man.
On second look, Davies did make contact with O'Hare but the City player did not go down immediately, which perhaps impacted the referee's thinking.
"We don't want to be known as the Sky Divers," said Robins.
"Callum doesn't dive he's the most fouled player in the division and there's contact there. He's shown me his ankle and there's a cut
"It's not a dishonest thing but he's got a booking."
Slavisa Jokanovic did not get a clear enough view of the incident, adding: "I was so far from this view, I did not see it."
The Blades could have won the game in the second half as David McGoldrick had two good chances while Morgan Gibbs-White saw his late shot well saved by former Blades goalkeeper Simon Moore.
The United boss felt his side were "nervous" in the first half before settling into the game after half time.
"Whether it's a fair result or not fair, it's not important," he reflected.
"In the first 45 minutes we were nervous and started to play long balls without having bodies in this situation.
"We didn't play good football but second in the 45 minutes we were a bit more comfortable and moved the ball better and created more."