"Oz" is the nickname for the Oswald State Correctional Facility, formerly Oswald State Penitentiary, a fictional maximum-security prison (level 4). The nickname is a reference to the classic film Wizard of Oz, which is notable for popularizing the phrase: "There's no place like home". In contrast, the series has used the tagline: "It's no place like home".
Many of the plot arcs are set in "Emerald City" ("Em City"), also a concept from The Wizard of Oz. In this experimental unit of the prison, unit manager Tim McManus attempts to emphasize rehabilitation and learning responsibility during incarceration, as opposed to pure punishment. Emerald City is an extremely controlled environment in which there is a carefully managed number of members of each racial and social group, with the hope of easing tensions among these various groups.
Under McManus and Warden Leo Glynn, the inmates in Em City all struggle to fulfill their own needs. Some fight for power; either power over the drug trade or power over the other inmate factions. Others, corrections officers and inmates alike, simply want to survive long enough. In particular the prisoners want to make parole or even to see tomorrow. The show gives a no-holds-barred account of prison life with all the plots, subplots and conflicts given context and explanation by the show's wheelchair-using narrator, Augustus Hill.
Oz chronicles the attempts of McManus to keep control over the inmates of Em City. There are many groups of inmates during the run of the show and not everybody within each group makes it out alive. There are the African American Homeboys (Adebisi, Wangler, Redding, Poet, Keene, Supreme Allah) and Muslims (Said, Arif, Hamid Khan), the Wiseguys (Pancamo, Nappa, Schibetta), the Aryans (Schillinger, Robson, Mark Mack), the Latinos (Alvarez, Morales, Guerra, Hernandez), the Irish (the O'Reily brothers), the gays (Hanlon, Cramer), the bikers (Hoyt), and a number of others (Rebadow, Keller, Stanislofsky). In contrast to the dangerous criminals, regular character Tobias Beecher gives a look at a normal man who made one fatal drunk-driving mistake. The episodes are narrated and held together by inmate Augustus Hill, who provides the show with some context, some sense of humor, etc.