Pollux’ Onomasticon is a Greek dictionary of the 2nd century CE. It is the first extant representative of the genre of the onomasiological dictionary in Greek. It attempts to organize the vocabulary of the Greek language into object domains and wordfields, and thus must resolve questions concerning the overall logical structure of the concepts (macrostructure) as well as the organization of the words cited under each concept (microstructure). In my paper, I examine together two devices used to produce structures in the Onomasticon: logical/descriptive categorizations, which are clearly object oriented, such as “people who construct X”, “people who use X“, “things to be found in X”, and which are used primarily to produce macrostructures; and the classification system of the parts of speech (PoS system), which is clearly word-oriented and which is used to produce microstructures. Despite the fact that these two types of categorizations are used primarily to produce structures belonging to different levels (macro- and microstructures respectively), there are cases in which they are mixed and combined on the microstructural level. I will focus on such cases and show that the combination between logical, object-oriented categories and parts of speech gives some insight into the logical importance that the parts of speech can have in an onomasiological dictionary. It can also explain a striking peculiarity in the PoS system Pollux uses, namely the regular appearance of the part of speech πράγματα (deverbal nouns) as distinctive from the part of speech ὀνόματα (nouns and adjectives).