When HBO was looking to expand its programming to include hour-long dramas in the late 1990s, the network was intrigued by writer/producer Tom Fontana’s pitch about a maximum security prison and a specific area, dubbed Emerald City, where prisoners could have more leeway in the hopes it would allow for their rehabilitation. Fontana came up with the idea following his work on Homicide: Life on the Street, where murderers were sent away: He wanted to explore what happened next.

Before The Sopranos or The Wire, television’s golden age arguably began on HBO on July 12, 1997, when the premium network premiered Fontana's prison drama Oz. As HBO’s first attempt at an hour-long dramatic series, it laid the groundwork for the dozens of risk-taking, novel, and novelistic shows to follow. On the series' 20th anniversary, check out some facts on the cast, the gore, and the alternate series finale idea that was never filmed.

1. Oz's creator is the person you see getting tattooed in the intro.

A former playwright, Fontana got his big break in television with the 1980s NBC hospital drama St. Elsewhere. In an impressive display of commitment to Oz—especially since he didn’t know if the show would even last beyond a season—Fontana volunteered his arm to get an “Oz” tattoo for the opening credits montage. The tattoo artist kept retracing his needle work so the crew could get the best take. Eventually, the artist stopped, saying that he “can’t let this guy bleed anymore.”

2. Oz's Greek chorus monologues were a necessity.

Viewers who tuned in to Oz were in for a shock—the show featured the kind of graphic violence and casual nudity you’d find in an actual prison. But they were also sometimes puzzled by Fontana’s narrative habit of putting one of the prisoners, Augustus Hill (Harold Perrineau), in front of the camera for fourth-wall-breaking soliloquies. Fontana said he chose this approach because “in prison, guys aren’t that forthcoming about what they think and what they feel because that leaves them open and vulnerable to attack ... so my thought was just to let someone articulate what all this craziness meant.”

3. Oz was filmed in a cracker factory.

Ernie Hudson, Terry Kinney, Harold Perrineau, and Eamonn Walker in Oz.Max Aguillera-Hellweg/HBO

To house the sprawling, 60,000-square foot prison set, HBO commandeered an abandoned National Biscuit Company (a.k.a. Nabisco) factory in Manhattan. (The building had been the first to mass-produce Oreo cookies for the company.) The space was obtained after Fontana couldn’t find any empty prisons in which to shoot.

4. Playing a Neo-Nazi in Oz made J.K. Simmons feel depressed.

Oz is probably best remembered for its sprawling ensemble cast, with actors like Chris Meloni, J.K. Simmons, and Perrineau all going on to successful careers; others, like Ernie Hudson and Rita Moreno, were already well-established. At the time, Simmons appeared to be having particular trouble inhabiting the repugnant skin of Vern Schillinger, the head of the prison’s Aryan population. Simmons referred to Schillinger in the third person and told The New York Times in 1999 that he became “depressed” as a result of the role. In an interview with NPR, Simmons also shared that fans would occasionally stop him in the street to let him know they endorsed Schillinger’s viewpoints.

5. Real ex-cons worked on Oz.

For realism’s sake, Fontana instructed his casting director to hire ex-cons as extras whenever he could. Not all of them were relegated to the margins: Chuck Zito, who had a recurring role as Italian mafia heavy Chucky Pancamo, was a then-member of the Hells Angels and had served six years in prison for various offenses. More notably, he received press coverage for allegedly knocking out Jean-Claude Van Damme at a strip club in 1998.

6. Tom Fontana didn't want to kill Simon Adebesi in Oz.

Dean Winters and Adewale Akinnuoye-Agbaje in Oz.HBO

From the first episode, Fontana made sure viewers didn’t grow too fond of any single character: One of the ostensible leads of the show, Dino Ortolani (Jon Seda), was murdered at the conclusion of the pilot episode, and the series picked prisoners off with regularity from that point on. But Fontana wasn’t trigger-happy when it came to killing off Simon Adebisi, the scheming, toothpick-munching inmate with a tiny hat sitting precipitously on the side of his head, who was played by Adewale Akinnuoye-Agbaje. “I didn't want to kill that character, but it was a necessity due to the actor's wanting to move on,” Fontana told CNN in 2003, “rather than me saying, 'This is the end of the story.'”

7. Adewale Akinnuoye-Agbaje exposed himself at random on the set of Oz.

Like many of the performers on Oz, Akinnuoye-Agbaje was expected to be comfortable with frontal male nudity—both his own and that of his castmates. According to Fontana, the actor didn’t appear to have many inhibitions about it. “If in a scene it said, ‘Adebisi takes out his penis,’ he would go, ‘I don’t take out my penis in this scene. There’s no reason for me to do that,’” Fontana told The Toast in 2015. “And I’d say ok, Adewale, don’t take out your penis. I don’t care. The next scene he’d take out the penis. It wasn’t scripted for that, but suddenly there was the penis.”

8. Oz predicted special musical episodes.

Remember the musical episode of Buffy, the Vampire Slayer? Or Scrubs? Oz did it first. With a cast taken in large part from the New York theater scene, the series was able to assemble an impressive all-song-and-dance episode in 2002. The highlight: Nazi Schillinger (Simmons) and nemesis Tobias Beecher (Lee Tergesen) in a duet.

9. There was a different ending planned for Oz.

After six seasons, Oz ended in 2003 with the surviving cast members being—spoiler alert—evacuated from Oswald State following a chemical attack. But Fontana originally wanted to do something else. He recalled reading about a prison town that once flooded, forcing inmates to work side-by-side with citizens to build sandbag barriers to protect the entire community. It was deemed too expensive to shoot.

10. Tom Fontana wouldn't let his mom watch Oz ... which was probably a good idea.

Despite her expressed desire to see her son’s work, Fontana told the press he was adamant that his then-75-year-old mother not watch Oz. “She said, 'I know a lot about what goes on in the world,’” Fontana said in 1997. “I said, 'You don't know about this.' This isn't a place I want my 75-year-old mother to go."