After graduating from the University of Arkansas, Miller, who is known as "The Big O" because of his large size (6-foot-9 and well over 300 pounds throughout his pro career), was selected by the Phoenix Suns with the 22nd overall pick of the 1992 NBA draft. He went on to play for various NBA teams over his eight-year career, including the Suns (1992–94, 1999–2000), Detroit Pistons (1994–95), Toronto Raptors (1995–96 and 1997–98), Dallas Mavericks (1996–97) and Sacramento Kings (1998–99). He was signed by the Indiana Pacers during the 2002 pre-season but was waived before the regular season began.
Miller was known as a talented Center with good passing skills, averaging 12.9 Points, 7.4 Rebounds, 2.9 assists, 1.9 blocks and 1.4 steals per game at his peak (1995–96). However, he was also notorious for his weight problems. In the beginning of his career, he was officially listed at 270 pounds, but in the later years of his NBA career he ballooned to 380 pounds, seriously reducing his mobility and stamina.
Following his initial stint in the NBA, Miller moved overseas, joining Iraklio BC in Greece for the 1998–99 season, before signing with the Kings in February 1999 to play in 4 games. He spent the next season back with the Suns, playing in 51 games. After the season, Miller became a bit of a basketball vagabond, joining the Harlem Globetrotters for the first of two stints with the team, then Pruszkow in Poland and back to the Globetrotters.
In December 2001, after another brief term with the Globetrotters, Miller was released for showing "no appreciation for what it takes mentally and physically to be a Harlem Globetrotter". In January 2002 he signed with Roseto of Italy but did not play there, and then joined the Continental Basketball Association with the Gary Steelheads before moving to the competing American Basketball Association's Southern California Surf and the USBL's Dodge City Legend. He returned to the Steelheads in October 2002, and was traded to the Dakota Wizards in February 2003.
In the 2003–04 season, Miller made a comeback with the Minnesota Timberwolves. Despite being listed at 315 lb, he was still able to play an average of 10 minutes a game off the bench in his last NBA season.
After a stint in Puerto Rico, Miller returned to the Wizards in October 2004 and next played with the Texas Tycoons in the ABA until February 2005. He then played in 2005 for the Arkansas RimRockers in the ABA.
After his stint in the ABA, Miller was signed to the Lawton-Fort Sill Cavalry of the Premier Basketball League. He was signed March 15, 2010 and released in December.
In April 2011, he was accused of pistol-whipping a man during an altercation at a barbecue cookout in Arnold, Maryland. He was arrested and charged with first- and second-degree assault, reckless endangerment, possessing a handgun, using a handgun in a violent crime, possessing a handgun in a vehicle and disorderly conduct, among other charges.
On November 1, 2011, Oliver Miller pleaded guilty in Anne Arundel County to first degree assault and possessing a handgun. He was sentenced on February 3, 2012 to a year in the Anne Arundel County jail (a five-year sentence with four years suspended), followed by five years of probation.
Miller said in May 2013 that he had moved to the Mesa, Arizona area the previous August and had been selling cars there for four months, working for former-basketball player Alvin Heggs. Miller was inducted into the University of Arkansas Sports Hall of Honor in September 2016 and was selected as an SEC Basketball Legend In January 2017.