The James Gordon actor, who is the first of the film's cast to react to the shootings, offered his "deepest sympathies" to the victims' families.
Oldman told TooFab in a statement: "My prayers and deepest sympathies are with the victims and their families of this horrific act."
Witnesses say a 24-year-old man named James Holmes wore a gas mask and body armour while opening fire at the Century 16 cinema in the city of Aurora in the Denver suburbs. He remains in police custody.
A 6-year-old child is among the dead and a 3-month-old baby was injured, say reports. Ten people are apparently still inside the cinema, with their conditions currently unknown.
US President Barack Obama and Warner Bros have already expressed their condolences to victims.
Other celebrities have also reacted to the story, with Jessica Simpson posting on Twitter: "Praying for the victims and families of those involved in the shooting in Colorado. My heart is broken. So awful. God Bless."
MTV presenter Mario Lopez wrote: "My thoughts & prayers go out to all the families of the victims of the senseless crime in Colorado. As a father, I'm devastated to hear... such a tragedy occur. Going to hug my daughter a little tighter tonight... God bless."
Police, meanwhile, have boosted their presence at tonight's premieres in New York.