O'Neal was born on March 6, 1972, in Newark, New Jersey, to Lucille O'Neal and Joe Toney, who played high school basketball (he was an All-State guard) and was offered a basketball scholarship to play at Seton Hall. Toney struggled with drug addiction and was imprisoned for drug possession when O'Neal was an infant. Upon his release, he did not resume a place in O'Neal's life and instead agreed to relinquish his parental rights to O'Neal's stepfather, Phillip A. Harrison, a career Army sergeant. O'Neal remained estranged from his biological father for decades; O'Neal had not spoken with Toney or expressed an interest in establishing a relationship.

On July 14, 2004, O'Neal was traded to the Miami Heat for Caron Butler, Lamar Odom, Brian Grant, and a future first-round draft choice (who would turn into Jordan Farmar in the 2006 draft). O'Neal reverted from (his Lakers jersey) number 34 to number 32, which he had worn while playing for the Magic. Upon signing with the Heat, O'Neal promised the fans that he would bring a championship to Miami. He claimed one of the main reasons for wanting to be traded to Miami was because of their up-and-coming star Dwyane Wade, to whom he gave the nickname "Flash". With O'Neal on board, the new-look Heat surpassed expectations, claiming the best record in the Eastern Conference in 2004-05 with 59 wins. He played in 73 games, his most since 2001 season, averaged 22.9 points a game along with 10.4 rebounds and 2.3 blocks. Shaq also made his 12th consecutive All-Star Team, made the All-NBA 1st Team, and won the Eastern Conference player of the Month award for his performance in March. O'Neal also narrowly lost the 2004-05 MVP Award to Phoenix Suns guard Steve Nash in one of the closest votes in NBA history.  Despite being hobbled by a deep thigh bruise, O'Neal led the Heat to the Eastern Conference Finals and a Game 7 against the defending champion Detroit Pistons, losing by a narrow margin. Afterwards, O'Neal and others criticized Heat head coach Stan Van Gundy for not calling enough plays for O'Neal. In August 2005, O'Neal signed a 5-year-extension with the Heat for $100 million. Supporters applauded O'Neal's willingness to take what amounted to a pay cut and the Heat's decision to secure O'Neal's services for the long term. They contended that O'Neal was worth more than $20 million per year, particularly given that lesser players earned almost the same amount.  In the second game of the 2005-06 season, O'Neal injured his right ankle and subsequently missed the following 18 games. Upon O'Neal's return, Van Gundy resigned, citing family reasons, and Pat Riley assumed head coach responsibilities. O'Neal later referred to Van Gundy as a "frontrunner" and a "master of panic." Many critics stated that Heat coach Riley correctly managed O'Neal during the rest of the season, limiting his minutes to a career low. Riley felt doing so would allow O'Neal to be healthier and fresher come playoff time. Although O'Neal averaged career lows (or near-lows) in points, rebounds, and blocks, he said in an interview "Stats don't matter. I care about winning, not stats. If I score 0 points and we win I'm happy. If I score 50, 60 points, break the records, and we lose, I'm pissed off. 'Cause I knew I did something wrong. I'll have a hell of a season if I win the championship and average 20 points a game." During the 2005-06 season, the Heat recorded only a .500 record without O'Neal in the line-up.  On April 11, 2006, O'Neal recorded his second career triple-double against the Toronto Raptors with 15 points, 11 rebounds and a career high 10 assists. O'Neal finished the 2005-06 season as the league leader in field goal percentage.

How did he do when he returned?