In music, Op. 42 stands for Opus number 42. Compositions that are assigned this number include:

Arnold – Homage to the Queen

Britten – Saint Nicolas

Bruch – Romance in A minor

Busoni – Berceuse élégiaque

Chopin – Waltz in A-flat major, Op. 42

Elgar – Diarmuid and Grania

Glière – Symphony No. 3

Haydn – String Quartet in D Minor, Op. 42

Holst – At the Boar's Head

Ippolitov-Ivanov – Caucasian Sketches, Suite No. 2

Mendelssohn – Psalm 42

Nielsen – Fynsk Foraar

Rachmaninoff – Variations on a Theme of Corelli

Rautavaara – Études

Schumann – Frauenliebe und -leben

Shostakovich – Five Fragments

Tchaikovsky – Souvenir d'un lieu cher

Widor – Symphony for Organ No. 5

Widor – Symphony for Organ No. 6

Op. 42