The veteran Okami (27-7) snapped his two-fight losing streak with a second-round TKO finish, earning his 11th UFC victory.
Roberts connected with several punches early in the first round. But by the midpoint, Okami regained his bearings and took control. He took Roberts down and maneuvered into top position. Okami unleashed a ground-and-pound flurry, but Roberts was saved by the horn.
Okami dominated round two. This time, when he got Roberts down and opened up his offense, Okami finished the job. Referee Herb Dean stopped the bout at 3:05.
Roberts (12-3), a Jackson's MMA fighter, was the second substitute in Saturday's fight with Okami. The Japanese vet was originally slated to face Luiz Cane, then Rousimar Palhares, but both withdrew with injuries.