The Manchester clubs go head to head in what has been billed as the Barclays Premier League title decider on Monday night, after which United face Swansea at home and Sunderland away while City visit Champions League-chasing Newcastle and host relegation-threatened QPR.
Mancini said earlier this week: "After the derby, United have two games which are not easy, but not difficult like us."
O'Neill's side defeated City 1-0 at the Stadium of Light in January and clinched a 3-3 draw at the Etihad Stadium at the end of March - the hosts scoring twice in the last five minutes to secure a point.
Quoted in the Daily Telegraph, the Northern Irishman said of Mancini's comments: "It took me by surprise because I had to check that the teams they [United] have to play beat Manchester City, like Swansea and ourselves.
"We have taken four points off Manchester City, but they have actually lost five against us this season. He had obviously forgotten to check the fixtures list properly.
"I will remember it for the next four years. I don't have an email address for him, but I really thought about sending him one to tell him to keep an eye on what he says in the future."
Whether Mancini's comments were merely aimed to rile Sunderland and get them to prove a point against United or not, O'Neill felt they showed a lack of understanding of the English game.
"I don't know what his insinuation was," he added.
"But if he had any idea about the league at all he would know that the integrity of the league is pretty good."
Source: PA
Source: PA