The Micillo-Males hypothesis proposes that the second part of The Third Grammatical Treatise (ca 1250) by Óláfr Þórðarson is based on a commentary of the insular tradition on Donatus’ third book of the Ars maior. This paper brings further support to the claim that the candidate suggested by Males (2018), the commentary called Ars Brugensis, is the model of the second part of the treatise. The relationship between the two texts is further analysed with regard to the changes that the author/translator Óláfr Þórðarson makes. The results establish that his treatment of Donatus’ brief precepts is faithful, but that the commentary is used more freely. Furthermore, the influence from the Doctrinale by Alexander de Villa Dei is shown to be greater than previously supposed. In trying to answer why Óláfr adapted aspects of the Doctrinale, I assess that part of the aim of the treatise was that it could be read in parallel with this work, making it likely that both texts were used in Óláfr’s school at Stafaholt. Óláfr’s comments that some grammatical figures are less useful for skaldic poetry, can be seen as an indication that these parts were meant to aid in the study of Latin grammar. The reordering of the content and the insertion of an additional grammatical figure from the Doctrinale are other important changes that would facilitate a parallel study.