Oldman, 59, was the front-runner for the best actor Oscar after sweeping awards season with prizes at the Golden Globes, the Screen Actors Guild, and from numerous film critics groups.
“I would just like to salute Winston Churchill, who has been marvelous company on what can be described as an incredible journey,” Oldman said while accepting his award.
He also addressed his 98-year-old mother, who he said was watching the ceremony from her sofa.
“I say to my mother, thank you for your love and support. Put the kettle on. I’m bringing Oscar home.”
Oldman has played Beethoven, Lee Harvey Oswald, Sid Vicious and Pontius Pilate in his 35-year acting career, but he has said taking on Churchill was by far the most daunting role.
“It fills you with fear, but maybe that’s what gives you the best work,” Oldman told Reuters last fall.
The London-born actor is completely unrecognizable in his role as the stooping, bow-tied Churchill, his own features transformed by a facial prosthetic and hours in makeup and costumes.
“Darkest Hour” focuses on May and June 1940 when Britain appeared on the brink of defeat in World War Two and Churchill faced deep divisions in his own government, the military and the monarchy.
Oldman won despite 2001 domestic violence allegations that resurfaced as the actor’s Oscar prospects gathered steam. Goldman has denied the accusations, made by his then-wife during divorce proceedings, and was never charged.
Oldman was previously Oscar-nominated for 2011 spy movie “Tinker Tailor Soldier Spy.” He is perhaps best known internationally for playing Sirius Black in the “Harry Potter” movies, and other work includes “The Dark Knight,” “Sid and Nancy,” and “True Romance.”