Entertainment Weekly has published a ten-page tribute marking the first anniversary of the star's death, which contains accounts of Ledger by those who worked with him.
The Patriot co-star Gibson remembered his friendship with the actor, saying that they would "hook up from time to time and chat".
He added: "He did ask me a couple of times for advice...But he was his own man. He was going to do what he was going to do."
Oldman praised the star's performance in The Dark Knight, saying: "I called a friend [on the first day of filming] and he said, 'How's Heath?' I said, 'He's breathtaking. He's going to be astounding.' I could tell just working for five minutes with him."
However, according to American Beauty star Wes Bentley, who was one of Ledger's closest friends, the actor often failed to appreciate his own talent.
"Heath didn't give himself enough credit for his talent as an actor...I wanted to explain to him, 'Please wake up to how great you are.'
"Heath's fun clouded its way through a room. It was infectious."
Other stars who contributed to the magazine included Ten Things I Hate About You co-star Julia Stiles and The Imaginarium of Dr. Parnassus director Terry Gilliam.
Ledger passed away on January 22 last year from an accidental overdose of prescription drugs.