Osi (tribe)
The Osi or Osii were a Roman era tribe dwelling north of the Marcomanni and Quadi, in a woody and mountainous country in or near present day Slovakia. Their national customs, as well as their language, were the same as those of the Aravisci, who Tacitus categorized as Pannonians. They paid tribute to the Quadi and Sarmatae. The exact districts they inhabited cannot be determined, nor do we know whether they had migrated from Pannonia, or whether they were an ancient remnant of Pannonians who once live in those districts. They are known to scholars from a small number of remarks by Roman authors including Tacitus.
Evidence from Tacitus
Tacitus described the Osi as living "behind" the Marcomanni, moving north from the river Danube. They were neighbours of the Marsigni, Gotini, and Buri.Tacitus described them as a Germanic "natio", using that term in a geographical sense.
In another passage, Tacitus explained that culturally and linguistically the Osi were nevertheless not really Germani.