Oz (Sat. (12), 11:30 p.m.-12:30 a.m., Mon. (14), 11 p.m.-midnight, HBO) Filmed in New York City by Levinson/Fontana Co. in association with Rysher Entertainment. Executive producers, Barry Levinson, Tom Fontana; co-executive producer, Jim Finnerty; co-producer, Debbie Sarjeant; supervising producer, Bridget Potter; directors, Darnell Martin, Nick Gomez; writer, Fontana; camera, Jean de Segonzac; supervising editor, Cindy Mollo; art director, Gary Weist; music, David Darlington, Steven Rosen; casting, Alexa Fogel. Cast: Ernie Hudson , Harold Perrineau, Terry Kinney, Eamonn Walker, B.D. Wong, Rita Moreno, Tony Musante, Leon, Lee Tergesen, Sean Whitesell, J.K. Simmons, O.L. Duke, Edie Falco , Kirk Acevedo, Dean Winters, Lauren Velez, Michael Gilhooly, Jon Seda, Zeljko Ivanek. Just how vicious and raw is “Oz,” HBO’s first drama series? So much so that the typically bold pay cabler is debuting it in the ungodly timeslot of Saturday at 11:30 p.m., leaving the impression that even HBO is a little squeamish about launching a prison drama in which sadistic white supremacists co-exist alongside cannibalistic parent killers. Just the kind of thing you want to come home to after the perfect date. Time period notwithstanding, “Oz” is the kind of brash, unsparing, disturbing ensemble series we would expect from Tom Fontana, the “Homicide: Life on the Street” guru who executive produces (with partner Barry Levinson) and writes all eight installments of this urban nightmare come to life. In depicting what it’s like inside an experimental unit of the fictitious Oswald Maximum Security Prison, Fontana — at last released from the shackles of broadcast primetime — serves up a bleak, agonizingly downbeat and occasionally over-stylized vision of prison existence. It’s about as pretty as a decaying corpse, and there is no one to root for. As such, series pushes TV’s content envelope in daring ways. And talk about your Not Ready for Prime Time Players. In the pilot, “The Routine,” we meet Donald Groves (Sean Whitesell), a sociopath who killed his parents and then ate them; Dino Ortolani (Jon Seda), a hair-trigger homophobe with a penchant for beating nearly to death any guy who propositions him; Schillinger (J.K. Simmons), a neo-Nazi who takes a frightened new inmate named Tobias Beecher (Lee Tergesen) as his love slave, tattooing a swastika onto his buttocks; and Jefferson Keane (Leon), who likes to pummel people just for the heck of it. By comparison, “NYPD Blue” seems as tame as “The Blue Lagoon.” Overseeing this jungle of retribution, anxiety, homemade knives (“shanks” in the prison lexicon) and misplaced idealism is warden Leo Glynn (Ernie Hudson), a by-the-book bureaucrat, and Tim McManus (fine work from Terry Kinney), the head man inside the penitentiary unit known as Em City (as in the land of Oz), who clings tightly to his odd notions of rehabilitation. Rita Moreno pops up in the second hour, “Visits, Conjugal and Otherwise,” as Sister Peter Marie, a psychological counselor and conjugal-visit arranger. One of the most powerful moments in the brutally intense “Oz” comes in that second episode, when Schillinger humiliates his “woman” Beecher by making him ask permission to have conjugal relations with his visiting wife. It’s gut-wrenching stuff. On the other hand, several elements of “Oz” fail to ring true. One is the very idea that a white-collar attorney like Beecher, who was convicted of manslaughter in a drunken-driving incident, would be placed into this high-voltage prison population. Another annoying and altogether unnecessary element finds a wheelchair-bound inmate named Augustus Hill (Harold Perrineau) delivering self-admiring, loosey-goosey commentaries directly into the camera for no reason other than to appear hip. We also don’t need quite as much of the herky-jerky film technique that Fontana employed with such success in “Homicide”; it tends to grate after a while. Yet there is also plenty that Fontana and company get right. The sharp racial divisions, the predatory homosexuality, the unceasing tension between guard and inmate, the tendency to prey on the weak-willed and the utter lack of trust and security are all dramatized in “Oz” with an uncanny level of believability. There is, then, a lot to admire about “Oz.” It looks and feels like the real thing. It is also so relentlessly ugly and grim that the simple act of viewing it weighs heavy on the soul.