Terry Kinney (Tim McManus) and Kathryn Erbe (Shirley Bellinger) were married in real life during the show. They divorced in 2006.
The arm that is being tattooed in the opening credits belongs to series creator Tom Fontana.
During the opening credits, the faces of all characters; specifically, main or recurring characters, are cut off by the camera so they cannot be identified.
The original idea was for the show to be about a medium-security prison. The title was supposed to be Club Med.
Is the first one-hour drama ever produced by HBO.
SPOILER: In the third season, the medical ward was privatized and taken over by a company called The Weigart Corporation. The Weigart Corporation is also the name of the company that bought St. Eligius Hospital in St. Elsewhere. Series creator Tom Fontana was a writer on St. Elsewhere.
The full name of the prison known as "Oz" was Oswald Maximum Security Penitentiary. The name was later changed to Oswald State Correctional Facility, Level 4
SPOILER: Kirk Acevedo (Alvarez) was absent from most of the first half of the fourth season because he was working on Band of Brothers. On the show, it was explained that Alvarez had escaped. Acevedo returned for the second half of the fourth season. But shortly after production began on the second half, he landed a recurring role on Third Watch and had to leave again. This time, it was explained that Alvarez had been put in solitary confinement. In some episodes, archive footage of Acevedo from past episodes was reused.
SPOILER: In the first part of the fourth season it was explained that Officer Diane Wittlesey (Edie Falco) moved away to London, GB. In real life, Falco left the show to star in the London production of the play "Side Man". She then went on to play Carmela Soprano in The Sopranos, another HBO production.
SPOILER: Christopher Meloni (Keller) left the show towards the end of the fourth season because he was also working on Law & Order: Special Victims Unit and he felt that he could no longer do both shows at the same time. On the show, it was explained that Keller was transferred to another prison. But Meloni missed Oz and decided to return. He decided that being able to work on two great shows simultaneously was worth the physical and emotional toll.
Dr. Nathan learns that Nurse Carol Grace previously worked at St. Eligius which was the name of the hospital in St. Elsewhere. Tom Fontana was a writer on that show.
SPOILER: The 100-minute series finale was titled "Exeunt Omnes" which is Latin for "all go out". It is a theatre term indicating that all characters exit the stage.
Most of the actors who played inmates on the show, played police officers on other series.
SPOILER: With the exception of Jason Kramer and Jackson Vahue, none of the inmates who survived until the end of the series ever made it out of Oz. The ones who were paroled, Beecher and Poet, returned to the prison shortly after they were released. The two who escaped, Alvarez and Busmalis, were eventually recaptured.
The prison set was actually an old warehouse that Tom Fontana had revamped. Fontana was also able to move his production offices into the space. Fontana would renew the lease on the space one year at a time in case the show got cancelled. During production on the fourth season, Tom Fontana learned that the owner of the property signed off a 10-year lease to NY1, a local New York City news station. The fourth season finale was titled "Famous Last Words" and ends with a fiery explosion in the prison. This was so that the episode could also serve as the series finale in case Fontana was unable to find a new filming location for the show. Fortunately, Fontana found a similar looking warehouse in New Jersey he could use and the show was able to continue.
Tom Fontana named the prison Oswald after Russell Oswald, who was the real-life warden of Attica when the infamous prison riot broke out there.
Jefferson Keane is executed halfway through the first season because the actor portraying him, Leon, refused to commit to a series. When Leon asked Tom Fontana why he was being killed off, Fontana told him it was because he said he wouldn't commit to more than four episodes. Leon replied that he thought he'd been talked out of that and actually wanted to stay.
Some cast members that played prisoners have noted that, throughout the series, if you showed up late to the set, your punishment would be that your character would either die or be raped the next week.
Craig muMs Grant (Arnold 'Poet' Jackson) actually wrote all the poems he read in the series.
SPOILER: By the time of the fourth season finale, they didn't know whether they would be renewed and so Tom Fontana wrote two endings: one where Beecher got paroled and one where he didn't. If the show was canceled they would end on the happy note of Beecher being paroled. The show was renewed but Fontana liked the sequence of Beecher's parole so much that he used it as a dream sequence in the season finale.
Prior to the boxing scenes in season 3, series creator Tom Fontana asked the actors involved if they had experience boxing. All said yes, but it turned out that only Chuck Zito had ever done any actual boxing.
Chuck Zito (Pancamo) first auditioned as a Biker, having being a veteran member of the Hell's Angels Biker Club. However because of his complexion and heavy New York accent, Tom Fontana thought he'd fit in better with the Italians on the series.
Kareem Said's name before his conversion to Islam was Goodson Truman. Zahir Arif's name before conversion was Jerome Van Dyke.
In contrast to many screen depictions of prison (such as The Shawshank Redemption), none of the characters imprisoned in Oz are innocent of their crimes. Even Jason Kramer, whose conviction is overturned due to police misconduct is heavily implied to be guilty of the crime he was sent to prison for. Series creator Tom Fontana has said that this decision to only have guilty characters was a deliberate one in order to better highlight the series themes of redemption
The prisoner's inmate numbers begin with two digits indicating the year they were incarcerated and then a letter which is the first letter of their last name. The only exception is inmates whose last name begins with O, such as Ryan O'Reilly. Because prison officials believed that an O could be easily mistaken for a zero, they use the letter P for inmates whose last name begins with O.
After his sons' death, Vern Schillinger gets a tattoo that reads "Andrew & Hank RIP".
Throughout the series, Kareem Said is the only inmate to pronounce Vern Schillinger's last name correctly.