The move has opened up spots on the 40-man roster for infielder Rodolfo Castro and right-handed pitcher Max Kranick ahead of Friday's reserve list deadline. Neither Castro nor Kranick will be exposed to December's Rule 5 Draft.
Williams struggled in 2020, going 2-8 with a 6.18 ERA and 1.57 WHIP over 11 starts. The 28-year-old's best season came in 2018 when he posted a 14-10 record with 3.11 ERA, 1.18 WHIP, and 126 strikeouts in 170 2/3 innings of work.
Osuna, 27, owns a .241/.280/.430 career batting line with 24 home runs and 88 RBIs across 276 games in parts of four seasons with the Pirates.