In his first season with the Warriors, Spellman appeared in 31 of Golden State's first 32 games and supplied 6.6 points, 5.0 rebounds and 0.6 three-pointers per contest. He made his Warriors debut on opening night (Oct. 24), playing 11 minutes and finishing with eight points and five rebounds. He scored in double-figures for the first time on Nov. 6 in Houston, posting 13 points, seven rebounds, three assists and two steals in just 18 minutes. In Utah on Nov. 22, Spellman went for 18 points and five rebounds. On Jan. 4, Spellman scored a career-high 23 points and grabbed four steals in a loss to the Pistons. Then, from Jan. 10 to Jan. 14, Spellman started in three straight games and posted 13.0 points and 4.3 boards per game in that span. On Feb. 6, Spellman, along with D'Angelo Russell and Jacob Evans, was traded to Minnesota. Spellman did not appear in a game for the Timberwolves, though he saw action in seven contests for the Iowa Wolves of the G League. In 31.1 minutes per game, Spellman averaged 19.6 points, 7.6 rebounds, 3.7 assists, 1.3 steals and 1.3 blocks.

Omari Spellman was selected by the Atlanta Hawks with the 30th pick in the first round of the 2018 NBA Draft. As a rookie, he appeared in 46 games for the Hawks and three games for the G League Erie Bayhawks. Spellman started in 11 of his Atlanta Hawk appearances. The 6-foot-9 forward averaged 5.9 points, 4.2 rebounds and 1.0 assists per tilt at the NBA level. He shot 40.2 percent from the field, including 34.4 percent from beyond the arc. Spellman had four double-doubles as a rookie. On Nov. 7, Spellman posted as career-high 18 points against the New York Knicks. From Dec. 31 to Jan. 4, the Villanova product played in three G League games for Erie. Over those three contests, he averaged a double-double with 16.3 points and 12.3 boards per contest for the Bayhawks. During his NBA time, Spellman dished out four assists on two occasions. He had seven games with three or more steals-plus-blocks. Spellman's Hawks finished with a record of 29-53 and missed the postseason. The forward has one more year left on his rookie contract, which also includes a team option for an additional season.