You've heard of ''Cop Rock''? Well, get ready for ''Con Rock.'' That's right, the ballsy HBO prison drama ''Oz'' is doing a musical episode. ''I got this call asking me, 'Would you sing a song?' and I said, 'What the hell are you talking about?''' remembers Rita Moreno, whose character, Sister Peter Marie Reimondo, opens the show by crooning Janis Ian's ''Days Like These.'' ''Then they told me the idea, and I just flipped.''
The idea for the extravaganza, entitled ''Variety'' (premiering Feb. 10), came about because Harold Perrineau, who usually narrates the series as inmate Augustus Hill, needed to take an episode off to film a part in the ''Matrix'' sequels. So creator Tom Fontana decided to replace his segments with musical-fantasy interludes expressing the innermost feelings of various denizens of Oswald State Correctional Facility. Thus, biker Jaz Hoyt (Evan Seinfeld) growls Todd Rundgren's ''No World Order,'' and Father Ray Mukada (B.D. Wong) reveals a kinky side with a suggestive rendition of Tori Amos' ''Leather.'' The unquestionable high note, however, is ''The Last Duet,'' a kitschy disco artifact originally recorded by Barry Manilow and Lily Tomlin and re-created by the eternally beleaguered Tobias Beecher (Lee Tergesen) and his white-supremacist tormentor Vern Schillinger (J.K. Simmons). ''I was Barry, of course,'' Simmons says. ''My bee-yotch was singing Lily's part.''
Music supervisor Chris Tergesen (Lee's older brother) was charged with finding the tunes to suit each character. ''The great thing about this show is everybody's on board for everything,'' he says. ''Obviously, there's some edgy stuff coming off those script pages, and pretty much to a person, everybody we asked to sing was like, 'Okay, let's roll the dice.''' The odds of success were increased by the performers' musical backgrounds, ranging from Moreno's Oscar-winning role in ''West Side Story'' to Seinfeld's full-time gig as lead singer of the heavy metal band Biohazard.
After shooting ''Variety'' last summer, Fontana found out that ''Buffy the Vampire Slayer'' was planning its own showstopper of an episode, which aired in November. ''I was like, 'Oh, f---, we're gonna look like the second guys in,''' he says. ''But ours is so different than theirs that I don't think it's really gonna hurt.'' And even though musical numbers permeate such other shows as Fox's ''Ally McBeal'' and Julia Louis-Dreyfus' upcoming NBC lounge-singer comedy, ''Watching Ellie,'' don't accuse Fontana of jumping on the bandwagon: ''I would hate to think it's a trend,'' he says. ''I guess we're all getting in touch with our inner Broadway diva.''