The Osi or Osii were an ancient tribe dwelling beyond the Quadi, in a woody and mountainous country. But their national customs, as well as their language, were those of the Aravisci, who Tacitus called Pannonians. They were, moreover, tributary 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 in those districts.
...however in that case Tacitus uses the term "natio" to define the Osi in a geographical way, as shown by his other mention of them...
The ethnicity of the Osi is a matter of debate: some claim they were Germanic, others Celtic, others Dacian. But most likely the customs, manners, language and ethnicity were a mixture of two of them or all of them as was common in those areas in Central Europe and the Balkans.
This article incorporates text from a publication now in the public domain: Smith, William, ed. (1854–1857). "Osi". Dictionary of Greek and Roman Geography. London: John Murray.