This is Oswald State Correctional Facility -- Oz -- home to some of the most violent offenders ever to appear on your TV screen. Simon Adebisi infected another prisoner with AIDS-tainted blood and spiked an inmate's food with ground glass, causing the internal bleeding that slowly killed him. Ryan O'Reily instructed his brain-damaged brother to kill the prison doctor's husband after she rejected his advances. Kareem Said, head of the black Muslims, instigated a riot that left several people dead. And so on. 

For three seasons (the fourth begins July 12), HBO has put this brutality on display in Oz, easily the best, most intense drama on television. Oz is ugly, frightening, an unflinching demonstration of the evil men do. 

And on a glorious day back in mid-March, I visited the Oz set, where the cast was shooting a scene that neatly summarizes the series' dark brilliance. A new inmate has been buying large amounts of drugs. The prisoners suspect he may be an undercover cop, so they put him to the test by forcing him to snort heroin -- something an undercover cop wouldn't be allowed to do. They lay out four lines on the floor of the weight room. After each line goes up his nose, he looks up to hear an ominous command from the prisoners who surround him: "More." It's like a Greek chorus of doom. The scene is shot over and over -- for more than an hour, from perhaps a dozen angles -- to capture the terror of the suspected officer and the satisfied glee of his antagonists. When it's wrapped up, there are smiles and laughter all around. 

If anything is jarring about being on this set, it's not the size of the glass-and-chrome "Emerald City" area, which looks more spacious on TV. No, it's the abrupt way the atmosphere changes. Prisoners who inflict the worst physical and psychological damage on each other one minute revert to what they are -- actors who like each other and appreciate the chance to work with great material. 

"It's a dream role," says Dean Winters, who plays the conniving O'Reily, standing on the roof of the building for a cigarette break, speaking with the same nervous energy as his character. "Every day I come to work, I get to be so nasty and I get to really exorcise my dark demons. "I was talking to someone the other day and they said, 'Do you find yourself starting to act like Ryan O'Reily in real life?' First of all, if I did that, I'd be in a lot of trouble. But I said no, it's just the opposite. I get to come here and be (a jerk) and get my ya-ya's out. Then, when I leave the set, I'm like the nicest person in the world." 

That doesn't stop people from being scared. Winters tells the story of being recognized on a subway early this year. He says the woman who saw him started whimpering. His response: "I was like, 'Relax, it's just a TV show, lady.' " 

But Oz has that effect. The show isn't real, but it feels that way. Every cast member has a story about viewers who've forgotten they're watching scripted drama. Lee Tergesen plays Tobias Beecher, an alcoholic lawyer sentenced to Oz after he killed a little girl in a drunken-driving accident. Beecher is Everyman in prison -- small, scared and vulnerable. Over three seasons, he has been raped, branded with a swastika tattoo and had his arms and legs broken. He also has learned to survive. Tergesen was in a New York pizza place when a man walked up and said he hated Beecher. "What he hated," Tergesen says, "was the things I did. When Beecher would be abused, the crew guys were always, 'Why don't you just kick his ass?' And I was like, 'It's a scene.' "

He surmises that people who watch Oz put themselves in the characters' places and wonder whether they could survive. "The rules that apply on the outside don't apply in here," says Eamonn Walker, the British actor who plays the powerful, focused and distinctly American Minister Said (pronounced SI-eed). "The strongest survive, or the smartest. Those are the rules. So when you call somebody evil because they did something or they do something or they manipulate to get their way, they don't really have any other options."

Consider the scene where the inmates force heroin on the suspected undercover cop. Walker has been watching on a monitor in the prison cafeteria (which is directly outside the prison gym/basketball court) as the prisoners impose their will. "In jail, there are all sorts of tests that you can put somebody through to see where they stand," he says. "That's a test. It's not evil. They don't have any other way of knowing. If they're wrong, he's got a bit of a problem, and they'll get him off it. If they make a big mistake, they've got so much too lose." 

Adewale Akinnouye-Agbaje agrees. He plays Adebisi, perhaps the toughest prisoner in Oz and one of the ones commanding "more." "I see him as a guy who's just doing his daily business, trying to survive before they knock him off," says Akinnouye-Agbaje, who knows something about people like the one he plays. He grew up in London and Lagos, Nigeria, which he describes as "more vibrant, more energetic, more ruthless than any city I've ever come across." A trained lawyer, model, actor and musician, he came to the United States seven years ago with the intention of making music inspired by his heroes, the great African musician Fela Kuti and the father of reggae, Bob Marley. Instead, he ended up in a Mary J. Blige music video and movies including Ace Ventura: When Nature Calls. Sitting in his dressing room, Adewale -- pronounced Ad-eh-wah-lay -- picks up a trumpet and blows a short, soulful solo. This is not what anyone would expect from Adebisi, who'd be more likely to use the instrument to crush someone's skull. 

Casting against type is something that appeals to Oz creator/executive producer Tom Fontana, which is why he hired venerable actress Rita Moreno -- the first person to win an Oscar, Tony, Emmy and Grammy -- to play Sister Peter Marie. She calls it "Tom's perverse notion of casting." As "Sister Pete," Moreno plays a small but meaty role trying to counsel the prisoners. Having made her fame as Anita in West Side Story, she's acquainted with theatrical violence. But Oz goes places the Sharks and Jets would have never talked about. Sometimes, that makes Moreno cringe. "I think some of it's excessive, I really do," she says. "Very likely, Tom may think so, too. But that's what he wants. I don't know what demons he's trying to exorcise, but I can't help feeling there's something that's very personal about the way he writes." That said, she finds his writing "innovative and bold in the best sense." Moreno tells what she calls a "delicious" story about Fontana calling her with an idea about how to stage the prison riot that ended Oz's first season. She said Fontana wanted to put on West Side Story with Beecher in drag as Anita. The simulated violence in the play would turn real, and the riot would be on. Unfortunately, Moreno says with a laugh, the rights to the play were unavailable. But that bit of mischief gives some insight into how Fontana thinks. 

Ernie Hudson, who plays warden Leo Glynn, says: "I don't know how people get through the stuff that goes on in Tom Fontana's head. And I hope to never find out. Hudson's career has included roles in everything from Roots: The Next Generation to Ghostbusters, and he smiles when he talks about the demons Fontana lets out through Oz. He also marvels at Fontana's ability to take the audience to prison, a place they don't necessarily want to go, and explore subjects they'd rather not face. "When I was a kid," Hudson says, "we thought the military was a place to grow up. With my sons, college was the place to grow up. It's that space between being a kid at home and being an adult on your own. . . . But unfortunately for a lot of young African-Americans, prison becomes that initiation period. I hate to think what that implies." 

While Oz deals with a heightened version of daily prison life, its subplots have gone to other corners of society -- male breast cancer, old people in prison, notorious inmates selling their belongings through computer auction services. And, this being a show about prison, Oz has delved into homosexual relationships. Among the more intriguing and talked-about story lines is the ongoing relationship between Beecher and Chris Keller (played by Chris Meloni). "When it started, we were really nervous about it," says Tergesen, who takes on the subject with characteristic good humor. "Obviously, just because it was something we hadn't really dealt with, especially at work. Because, let's face it, it wasn't like I hadn't kissed men before. But it was weird when we first got that script because we talked about it and we really wanted to go towards it rather than shy away from it. And I think it paid off." 

Before Oz, Tergesen's best-known role may have been as Terry, one of Wayne and Garth's headbanging friends in the Wayne's World movies. "I was the guy who said, 'I love you, Wayne. I love you, Garth.' Now when I say 'I love you' to men, it's a little different," he jokes. "But basically, the love theme is there in all my work." Tergesen also has a serious psychological take on what has happened to Beecher. "The greatest thing humans can do is assimilate," he says. "The abused child can see the abuse as love. So it's the same sort of thing -- you start to interpret things in a way that keeps you from losing your mind." 

Exactly right, says Winters, whose Ryan O'Reily keeps his sanity (and the breath in his body) by playing his fellow prisoners against one another. Winters grew up in New York City, which gives him a leg up on survival instincts. Seven years ago, he and his brother Scott (who plays his brother Cyril on Oz) met Fontana when they were bartending. Both made appearances on Fontana's acclaimed NBC show, Homicide: Life on the Street, before getting their roles on Oz. In the spring, Winters thought Oz would finish its 11 weeks of taping and he'd be out looking for work. But HBO ordered eight more episodes, which gave the cast and crew double the work. The second batch of episodes will begin airing in January, leading up to the next season of HBO's The Sopranos. The stories of Tony Soprano and the New Jersey mob have garnered their share of acclaim -- and then some. Oz hasn't gotten nearly that amount of recognition, and cast members speculate it's because of the grimness. 

"People know it's quality TV," Walker says. "They just wish it wasn't so real. When you explain that it has to be that harsh, it has to be that real, the violence has to be that bad because we're used to westerns where people go bang, bang, bang and John Wayne gets up again two seconds later and you watch him in the second matinee. "We've become conditioned to think that it's all right to watch all that violence and, therefore, we can watch it over and over again. The whole point of Oz is, yeah, it's harsh. Don't go outside and play with this stuff. You bleed. You die."