Johnson played high school basketball for Dorsey in Los Angeles. In his junior season, he came off the bench and averaged 11 points and 7 rebounds per game. As a senior, he averaged 18.3 points, 10.1 rebounds and 1.5 blocks per game, being named to the All-CIF LA first team. Led by Johnson, Dorsey reached the sectional semifinals of CIF LA.
College career
After considering offers from Wyoming, South Florida and Long Beach State, Johnson verbally committed to Oregon State. He stated that the fact that Corvallis is a college town was his motive behind choosing Oregon State, along with him feeling needed by his future teammates and having the opportunity to compete against top talent in Pac-10. He achieved a career-high performance in scoring with 19 points as a freshman, as the Beavers lost 76–63 to Arizona. He tied that feature as a sophomore, also recording his first double-double, with 10 rebounds, helping his team get past Nebraska with a narrow 64–63 win. Although 6-foot-7 when he started college, Johnson grew more than two inches before his junior season to reach a height of 6-foot-9. Coach Craig Robinson expected Johnson to become Oregon State's focal point in offense, but was mainly used as a reserve player due to his sub-par effort on the defensive end. Johnson had a breakout game in the 2010 Civil War rivalry game versus the Oregon Ducks, scoring a season-high 18 points on 4-of-6 three-pointers. In his senior season Johnson achieved a career-high with 13 rebounds, also adding 15 points for his second career double-double, as Oregon State lost 83–80 to Seattle. A few days later, he had a career-high 6 steals, also adding 13 points, as Oregon State lost 66–60 to Texas Southern. Prior to the game against local rivals Oregon, Johnson commented on his opponents that they are "a feisty little bunch of dudes.". Further commenting on the game he said "They bump us more, you get elbows in the back a little more than other games, it's just physically and verbally more intense.", but he also clarified "I wouldn't call 'em dirty". In four seasons with the Beavers, Johnson appeared in 120 games, starting 67 of them and averaged 6.4 points and 4.0 rebounds per game.
After graduating from college, Johnson was selected by the Oshawa Power to play in their 2011 pre-season squad. He played in Oshawa's inaugural game in the NBL Canada, posting 8 points and 3 rebounds. Against the Halifax Rainmen he finished with career-highs in both scoring and rebounding with 37 and 16 respectively; his scoring performance being a then NBL Canada record. In the wake of the loss Johnson commented: "We weren't guarding at all and had too many turnovers". In the D-League 2012 draft, Johnson was selected 74th overall by the Canton Charge, only to be waived a few days later.
After being named in the All-NBL Canada second-team and an All-Star in the 2011–12 season, Johnson signed with the Summerside Storm in December 2012. He posted a season-high 34 points, along with 13 rebounds, to help the Storm overcome his former team Oshawa Power. In his two seasons in Canada, Johnson started 43 of his 57 regular-season games, averaging 18.5 points, 8.5 rebounds, 2.6 assists, 1.4 blocks and 1.4 steals per game.
In September 2013, Johnson signed with ACB team CB Valladolid. In Spain, Johnson had his best scoring performance against FC Barcelona Baloncesto, scoring a game-high 20 points. Johnson started 24 of the season's 34 games, averaging 11.6 points and 6 rebounds per game in 26 minutes playing time.
Johnson was selected 39th overall in the 2014 NBA D-League Draft by the Maine Red Claws. In February 2015, playing against the Westchester Knicks, Johnson had a season-high performance of 30 points, also having 15 rebounds. He tied his scoring season-high two weeks later, against the same opponent, grabbing 14 rebounds to achieve a double-double.
2015–2016 seasons
On September 25, 2015, Johnson signed with the Portland Trail Blazers. However, on October 23, he was waived after appearing in four preseason games. On October 31, he was reacquired by the Red Claws.
On November 9, 2016, Johnson signed with Neptūnas Klaipėda of the Lithuanian League. On February 10, 2017, he parted ways with Neptūnas due to disciplinary issues.
On April 6, 2018, the Memphis Grizzlies announced they had signed Johnson to multi-year contract. He would make his NBA debut before the end of the 2017–18 season. On June 24, 2018, he was released by the Memphis Grizzlies.
On September 21, 2018, Johnson signed with the Indiana Pacers. He was waived on October 11, 2018, by Indiana. Omari will play in 2018/2019 for the Fort Wayne Mad Ants who acquired his G-league rights in a trade with the Memphis Hustle.
Johnson was added to the Fort Wayne Mad Ants training camp roster. On February 23, 2019, Johnson recorded a career-high 8 3-pointers as he recorded 28 points, 6 rebounds and 2 assists in a 99-100 losing effort to the Canton Charge.
Personal life
The son of Jennifer Gordon and Dave Johnson, he hails from Kingston, Jamaica. Both of his parents are former basketball players, his mother at Florida Tech, and his father at Shaw University. Johnson has two brothers, Shakir and Kahlil.
