During Diallo's second season in the league and with the Thunder, he averaged career highs in points (6.9), rebounds (3.6), assists (0.8) and steals (0.8) per game. Diallo secured his first of three double-doubles during the fifth game of the season, Oct. 30 against the Trail Blazers, recording 12 points, 10 rebounds, two steals and one assist in 20 minutes. He would achieve his other two double-doubles Jan. 25 in a nine-point win over the Timberwolves and Aug. 14 against the Clippers. The 6-foot-5 wing blocked a season-high two shots during an eight-point win over the Magic on Nov. 5, also posting six points, six rebounds, two steals and one assist in 22 minutes. Shortly after, Nov. 10 against the Bucks, Diallo swiped a career-high three steals to go along with 10 points in 22 minutes. Against the Nuggets on Aug. 3 in the NBA bubble the sophomore dished a career-high four assists in 24 minutes, also posting two points, seven rebounds and one steal. On Aug. 14 against the Clippers, Diallo scored a career-high 27 points and grabbed a career-high 11 rebounds, plus one assist and one block in 40 minutes. During seeding play overall, Diallo averaged 10.6 points, 5.6 rebounds, 1.8 assists and 1.1 steals in 24.6 minutes. He also appeared in three of the Thunder's seven playoff games against the Rockets, totaling 13 points, six rebounds, two blocks and one assist in 25 minutes.

Before making his NBA debut, Hamidou Diallo was traded twice during the 2018 offseason. He was drafted by the Brooklyn Nets and subsequently traded to the Charlotte Hornets before finally sticking with the Oklahoma City Thunder. He played 51 NBA games as a rookie and another six in the G League. In those six G-League starts, Diallo dominated. He averaged 19.2 points, 7.3 rebounds, 1.8 steals and 29.5 minutes per game. The University of Kentucky product averaged 3.7 points and 1.9 rebounds in 10.3 minutes per game at the NBA level. He recorded double-digit points five times, including in back-to-back games in December. His best production came in November and December. Over the course of those months, he racked up four double-digit scoring games, averaged 5.3 points and shot 51 percent from the field. Diallo did not appear in any of the Thunder's five postseason contests.