Black Jack: The two were once classmates as fellow doctors in training in college and have a good friendship as he often references patients in critical condition or with severe health problems to Black Jack, though warn them that he is twisted and a bit eccentric. Black Jack in turn seems to view him as a friend and a competent doctor as he often asks for his help in medical records or about certain medical conditions, assisting him in certain tough surgeries, and referencing certain patients and their backgrounds.
Osamu Tezuka is the real life author of Black Jack and makes a cameo throughout the anime series as a doctor who works at a general hospital and recommends patients with serious conditions that can't be cured by normal means to Black Jack. Black Jack has also asked for his help and consulted him on certain medical conditions, for certain medical records, assistance in certain surgeries, and to reference certain patients.
Osamu Tezuka usually inserts himself in all of his manga, anime, and works. He can be recognized by his round spotted nose, round glasses, and sometimes wearing a buret (but not in the anime, though he is frequently seen with his normally prominent features).
In the Black Jack specials that precede the 2004 anime, Osamu Tezuka made a cameo as a manga artist in Tom's coffee shop in the second special.