Ancient people of the middle Danube region, living north of the river behind the Marcomanni and Quadi, in the area of modern south-western Slovakia. Known almost entirely from Tacitus's Germania, which records they spoke the 'lingua Pannonica' and, with their neighbors the Cotini, paid tribute to the Quadi and the Sarmatians. Their ethnic affiliation (Illyrian vs. Celtic) is disputed among scholars.
Remained outside the Roman Empire but were progressively 'Dacianized' during the 1st and 2nd centuries AD, per Wikipedia — implying gradual cultural absorption rather than a sharp end date.
Primary source (Germania) — the near-sole surviving record of the Osi.