Short answers, pulled from the story.
Oium was a name for a fertile part of Scythia, roughly in modern Ukraine, where the Goths settled after migrating from Gothiscandza. Jordanes, writing around 551, places it near the Sea of Azov, though other scholars have proposed it corresponds to the Pripyat or Rokitno marshes near the modern Belarus-Ukraine border.
Filimer, son of Gadaric, was the Gothic king who led his people to Oium, described by Jordanes as approximately the fifth king since a founder named Berig. Under his leadership, half the Gothic army crossed a bridge into Oium before the bridge collapsed, stranding them in the new land. He subsequently led those who crossed to victory over a people called the Spali.
Most scholars interpret Oium as a dative plural of a Proto-Germanic noun reconstructed as *awjo, meaning 'well-watered meadow' or 'island'. The same root appears in the Latin name Scatinauia, recorded by Pliny the Elder, from which both Scandinavia and Scania take their names.
The Chernyakhov culture, also called the Santana de Mures culture, covering parts of Ukraine, Moldova, and Romania, corresponds to Gothic-influenced Scythia as described by 3rd and 4th century sources. The Wielbark culture of the Vistula Basin is proposed as representing the Goths before their southward migration.
Scholarly opinion is divided. Some historians believe the account preserves a genuine historical core, while others, including Walter Goffart, Patrick Geary, and Michael Kulikowski, argue it has little relation to actual history. A major problem is that Jordanes dates the Goths' arrival in Oium well before 1000 BCE, a chronology that even sympathetic historians do not accept.
Jordanes states he was summarizing a now-lost larger work by Cassiodorus. He also cites early Gothic songs and the writing of a chronicler named Ablabius, described as a famous and trustworthy recorder of Gothic history. Jordanes explicitly says he prefers these written sources over oral legends he encountered in Constantinople.