Los Blancos' midfielder says the club would struggle to replace the sheer number of goals the Portuguese has racked up in his time in Spain
The Portugual international revealed he was unhappy at the club last season, resulting in speculation he could return to former club Manchester United.
Madrid's pursuit of Tottenham star Gareth Bale has added to speculation the Welshman could be a replacement for the 28-year-old and Ozil says the club would be foolish to allow Ronaldo to leave the Santiago Bernabeu.
He told CNN: Cristiano Ronaldo is very important to the team, just look at the goals he has scored in the past few seasons.
"He's a great person off the field. I hope he can stay here for a long time because he can help us win important titles."
Ozil, 24, was part of a Blancos side that surrendered the Liga title to Barcelona last season, a failure that saw Jose Mourinho leave the club and replaced by Carlo Ancelotti, who arrived from Paris Saint-Germain.
And the Germany international says the Italian coach has wasted little time in changing the club's style of play as they go in search of a 10th Champions League success next term.
"Ancelotti has his own football philosophy," added Ozil.
"He wants us to play quicker than before. His system is different to what we're used to but that's why friendly games are important.
"We can get used to a new way of playing. We have quick attacking players so I believe we are equipped to adopt his system successfully."