Officially, the text for these movements is part of the Sanctus; beginning in the late Renaissance, some composers began to set the Benedictus as a separate movement, perhaps because the text does seem to hold a gentler mood (or Affekt) than the Sanctus.
In many setting of the Mass, the Osanna is often accompanied by a change of mood, key, instrumentation, or scoring, as Bach does here. Here, Bach uses a strict da capo form (ABA), in which the two movements setting “osanna in excelsis” are identical. Bach even writes “Osanna da capo” in the score as a space saving device, rather than writing out the music twice. This is a common form in Baroque music, though we most often associate it with arias. (Bach employs several da capo arias in his Passions).