Os justi ('The mouth of the righteous'), WAB 30, is a sacred motet composed by Anton Bruckner in 1879.

[4][5][6] While the first performance was expected on Traumihler's name-day (31 July 1879), it finally occurred four weeks later on 28 August 1879 on the feast of Saint Augustinus.

[7] The work was first edited by Theodor Rättig, Vienna in 1886, together with three other graduals: Locus iste, Christus factus est and Virga Jesse.

[8][7] The completed setting with the extra verse (Inveni David) and the repeat of the Alleluja – the manuscripts of which are archived at the Österreichische Nationalbibliothek[9] – is put in Band XXI/28 of the Gesamtausgabe.

[13] The last sentence, on "et non supplantabuntur" (bars 65-69), is sung pianissimo by the soprano, on a sustained tonic chord by the five other voices (ATTBB).

On 28 July 1879, Bruckner added an extra verse Inveni David scored for unison male voices with organ accompaniment, and a repeat of the 2-bar Alleluja.

[4] Traumihler was a fervent supporter of the Cecilian Movement; the reason why Bruckner composed this motet in Lydian mode, without any alteration in the key and in the whole score, and with large use of unaltered chords.