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.
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.
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.
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.
In contrast to many screen depictions of prison (such as The Shawshank Redemption (1994)), 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.
In the 2013 book "Difficult Men: Behind the Scenes of a Creative Revolution: From The Sopranos and The Wire to Mad Men and Breaking Bad", author Brett Martin notes that while he was a writer and executive producer on Oz, Tom Fontana would often subsidize Oz actors' outside theater projects. Fontana said that he gave the donations so that he wouldn't actually have to attend the shows--that it was "worth $1000 not to have to see a play."
The trivia items below may give away important plot points.
Christopher Meloni (Keller) left the show towards the end of the fourth season because he was also working on Law & Order: Special Victims Unit (1999) 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.
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.
Five prisoners are executed throughout the show's run and almost all with different methods: Jefferson Keane and Richard L'Italien die by lethal injection in Season 1; Donald Groves was executed by Firing Squad in Season 1; Shirley Bellinger is executed by hanging in Season 4; and Cyril O'Reilly dies in the electric chair in the series finale.
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 (1982). Series creator Tom Fontana was a writer on St. Elsewhere.
Kirk Acevedo (Alvarez) was absent from most of the first half of the fourth season because he was working on Band of Brothers (2001). 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 (1999) 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.
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 (1999), another HBO production.
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.