Roman itineraria, as well as late antique and medieval testimonia, also mention a settlement located on the river – Anasamus, Ansamon, Ἄσημα (n.pl.) or Ἄσημον.

The name is defined as Thracian by V. Tomaschek. St. Mladenov links Asamus with the appellative ἀσάμινθος and considers that the name is non-Indo-European, while based on the meaning “bathtub”, i.e. an object related to water, he concludes that Asamus simply means “River”. Nevertheless, he also allows for a Thracian origin. D. Detschew supports the association with ἀσάμινθος, rooted, according to him, in ασ- “stone”, derived from IE *ak’- “sharp, stone” as in Old Indian áçman – “stone, rock, sky” and Old Bulgarian “камы” (kamıi) – “stone”. This etymology is unquestionably accepted in following studies. V. Georgiev substantiates it by citing two facts as evidence for the reliability of the interpretation: 

1. the rocky bed of the upper course of the Osam River, its rocky right bank, and the presence of the river name Kamenica (Bulg. stony) in the vicinity of Osam, near the village of Ugarchin, which, according to him, is a kind of translation of Asamus; 

2. the modern Croatian name of the Dalmatian town of Asamum (a settlement name identical to the Bulgarian river name) – Lapad, from the medieval Latin Lapida < lapis “stone”. I. Duridanov and K. Vlahov shared his opinion.

To V. Georgiev’s arguments can be added the possible association of the river name with the root of the Thracian gloss ασᾶ “coltsfoot, tussilago,” the name of a plant which, because of its sharp leaves, is also thought to derive from the IE *ak’- “sharp.” It is possible that other Thracian toponyms were also formed from the same root – Ἀσαί, Ἄσσα, Ἄσσαρα, as well as the first component of the personal name Ασαβιθυς.

The modern name Osam is a development of the ancient name after its adoption by the Slavs (Осмъ/ Osmı, attested in the 15^th century). The transition from short [a] to [o], characteristic of the Old Bulgarian language, shows that Asamus belongs to the oldest names borrowed from Thracian into Bulgarian – from the period between the 6^th and 8^th centuries.