Five points separate the two rivals ahead of Sunday's Classique, and the Italian believes that Elie Baup's side have improved a lot over the course of the season.
“OM are a strong team,” Ancelotti told reporters.
“They are an aggressive side and they have gained a lot of mental strength [this season].
“The championship of France is a special one, it has a lot of rhythm, speed and intensity and it is not always easy for us."
The Italian also admitted he feels that his attacking trio Javier Pastore, Lucas Moura and Ezequiel Lavezzi must also learn how to help the team at the back if they are to prosper.
“In Ligue 1, talent is not enough. The defensive effort is the most difficult thing for Pastore, Lavezzi, Lucas to learn.”