The touring Lions have been left dismayed that neither of the New Zealand players was found guilty of foul play.
O'Sullivan said: "Brian had a call from him and he just kind of commiserated with him being injured as if he had nothing to do with it which I find strange."
Umaga has faced criticism all week from Sir Clive Woodward's men and O'Sullivan is the latest to speak out about the actions of the New Zealand centre.
"Brian O'Driscoll's tour is over and it ended in a very bad way," said Woodward's right-hand man.
"We can park it there if you want. Things do happen in battle and I accept that. The incident was not premeditated but I think it was opportunistic and I think he knew exactly what he was doing."
The Lions also called into question Umaga's sportsmanship, saying he should have at least offered his best wishes as O'Driscoll was carted off the field.
The tourists are also seething that the All Black captain has yet to say sorry for what the Lions deem was a possible career-threatening tackle.
"He hasn't apologised which is upsetting," added O'Sullivan. "There are two things here which Brian is upset about.
"One is that when he was lying on the ground the New Zealand captain didn't approach him to see how he was which is a long tradition in rugby going back to the amateur days. The captain always enquired about an opposition player's well being.
"To ring up and say he is sorry that he got hurt without offering an apology is disappointing too. It's like kicking a ladder from under somebody then commiserating he had an accident on a ladder. He was part of it.
"I think Brian is still upset about that and rightly so. You can't change what happened in the cold light of day but he could have handled that better as New Zealand captain."
O'Sullivan continued: "We've all had our take on it and this Lions tour will be remembered for this incident so whatever happens from here on in will blight the whole Lions tour and that is unfortunate.
"And even after the event I think it could have been handled a little bit better in retrospect. It's been disappointing."