The O salutaris hostia setting originates as the second part of a motet by Weerbeke that survives in MilD 1 (Ave regina celorum) and MilD 2 (Ave domina angelorum) as the fifth item in the cycle of motetti missales that begins with Ave mundi domina. The work was composed before 23 June 1490, the date entered in MilD 1 by Gaffurio. A probable terminus ante quem is 1480/1, when Weerbeke left Milan for Rome; nothing stylistically in the cycle makes it seem later than those by Compère of the 1470s, and Weerbeke only returned to Milanese service in 1489. No matter which date, this is probably the oldest composition in the Occo Codex. Within the cycle, the motet assumes the role of the Sanctus. The first part of the motet is polyphonic; the second part, coinciding with the Elevation of the Host, is homophonic. The two Milanese sources have coronae over each note to emphasise the solemnity of the occasion.¹

The only deviation from the strict homophony occurs in the Contratenor, mm. 27-28, which is different in all three sources. MilD 1 has semibreves C and F in m. 27, obviously to prevent parallel fifths from arising between Contratenor and Bassus. These are erroneously notated as breves in MilD 2. In the Occo Codex, the semibreve movement is extended to m. 28. The awkward leap of a seventh in the Bassus (mm. 12-13) can similarly be explained as an evasion of parallel octaves with the Contratenor.

In each source, the Bassus contains a different number of b-molle signs lowering E to E-flat. The one all three sources agree on is in m. 22, creating an interesting false relation with the E-natural in the Tenor in the next bar.

The melody of Weerbeke's Tenor appears to be derived from a cantus prius factus: the same tune underlies Obrecht's song Laet u ghenoughen (transmitted without text in its only source), and also in a number of German sources of the 16th century with different texts.²

Notes
[1] Bonnie J. Blackburn discusses this piece in detail as a representative of a "devotional style" characterized by four-part chords consisting of breves with fermatas in "The Dispute About Harmony c. 1500 and the Creation of a New Style," in Théorie et analyse musicales 1450-1650, ed. Anne-Emmanuelle Ceulemans and Bonnie J. Blackburn (Louvain-la-Neuve: Université catholique de Louvain, 2001), pp. 13-16.
[2] See Jacob Obrecht, Secular works and textless compositions, ed. Leon Kessels and Eric Jas, New Obrecht Edition 17 (Utrecht: KVNM, 1997), p. xxxvii. Reference kindly provided by Eric Jas.