The Cast is headed up by Ernie Hudson as the Warden. Other characters include, Tim McManus, the brain behind Em City, who is involved with one of his guards, Wittlesly. Sister Pita Marie is the spritual guide of most of the inmates, and taken more seriously than the priest (BD Wong). Over the past two seasons, OZ has gone through many deaths, a long and painful poisioning of one of the first cast members, and, as a cliffhanger for the end of hte first season, a riot in Em City. The show is filmed in an enormous abandoned warehouse in New York City, and was converted into almost a small prison for filming. 
The best cast member (in my somewhat oversexed girl opinion) is Dean Winters, who plays Irish bad boy Ryan O'Reily. [OH my god...sorry. I just realized how romance novel that sounded] Sentenced to life, but up for parole in 12, Ryan has had breast cancer, fallen in love with his doctor, and convinced his slow brother Cyril to kill her husband. But he looks great in a wife beater.
For more info on OZ, check out the website. It rambles far less than I do and goes into more detail.
New episodes of OZ premiere Wednesday nights at 10:30