In music, Op. 31 stands for Opus number 31. Compositions that are assigned this number include:

Alkan – Preludes, Op. 31

Barber – Summer Music

Beethoven – Piano Sonata No. 16

Beethoven – Piano Sonata No. 17

Beethoven – Piano Sonata No. 18

Britten – Serenade for Tenor, Horn and Strings

Chopin – Scherzo No. 2

Elgar – A Song of Flight

Elgar – After

Enescu – Vox Maris

Farrenc – Piano Quintet No. 2

Górecki – Symphony No. 2

Gottschalk – Souvenir de Porto Rico

Prokofiev – Tales of an Old Grandmother

Rachmaninoff – Liturgy of St. John Chrysostom

Saint-Saëns – Le rouet d'Omphale

Schillings – Mona Lisa

Schoenberg – Variations for Orchestra

Schumann – 3 Gesänge

Tchaikovsky – Marche slave

Vierne – Vingt-quatre pièces en style libre

Op. 31