Oium was a name for Scythia, or a fertile part of it, roughly in modern Ukraine, where the Goths, under a legendary King Filimer, settled after leaving Gothiscandza, according to the Getica by Jordanes, written around 551.
It is generally assumed that the story reproduced by Jordanes contains a historical core, although several scholars have suggested that parts of it are fictional.
Name etymology
Jordanes does not give an etymology, but many scholars interpret this word as a dative plural to a noun, widespread in the Germanic languages, whose Proto-Germanic reconstruction is *awjō and which means 'well-watered meadow' or 'island'. (The same noun is also found in Scatinauia, the Latinised name of an island in Northern Europe mentioned in Pliny the Elder's Naturalis historia, from which the names of Scandinavia and Scania originate.) This noun is generally derived from the Proto-Germanic word *ahwō 'water; stream, river' (whence Gothic 'river'), which is cognate with Latin aqua 'water'. This is seen as consistent with the description Jordanes gave of the Goths delight in this region's fertility.
As mentioned for example by Dennis H. Green Jordanes describes another place with a similar name — the place where the Goths' relatives the Gepids lived: XVII (96) These Gepidae were then smitten by envy while they dwelt in the province of Spesis on an island surrounded by the shallow waters of the Vistula. This island they called, in the speech of their fathers, Gepedoios [emphasis added]; but it is now [in the 6th century] inhabited by the race of the Vividarii, since the Gepidae themselves have moved to better lands. The Vividarii are gathered from various races into this one asylum, if I may call it so, and thus they form a nation. (97) So then, as we were saying, Fastida, king of the Gepidae, stirred up his quiet people to enlarge their boundaries by war. He overwhelmed the Burgundians, almost annihilating them, and conquered a number of other races also.
Chronology
A problem with Jordanes' account is that he dates the arrival of the Goths in Oium well before 1000 BCE (approximately 5 generations after 1490). Historians who accept Jordanes' account as partially reflecting real events do not accept this aspect.
Jordanes
Mierow's translation of the one short passage in Getica IV, which mentions Oium is as follows: [...] But when the number of the people increased greatly and Filimer, son of Gadaric, reigned as king — about the fifth since Berig — he decided that the army of the Goths with their families should move from that region. (27) In search of suitable homes and pleasant places they came to the land of Scythia, called Oium in that tongue. Here they were delighted with the great richness of the country, and it is said that when half the army had been brought over, the bridge whereby they had crossed the river fell in utter ruin, nor could anyone thereafter pass to or fro. For the place is said to be surrounded by quaking bogs and an encircling abyss, so that by this double obstacle nature has made it inaccessible. And even to-day one may hear in that neighborhood the lowing of cattle and may find traces of men, if we are to believe the stories of travellers, although we must grant that they hear these things from afar. (28) This part of the Goths, which is said to have crossed the river and entered with Filimer into the country of Oium, came into possession of the desired land, and there they soon came upon the race [gens] of the Spali, joined battle with them and won the victory. Thence the victors hastened to the farthest part of Scythia, which is near the sea of Pontus; for so the story is generally told in their early songs, in almost historic fashion. Ablabius also, a famous chronicler of the Gothic race, confirms this in his most trustworthy account. (29) Some of the ancient writers also agree with the tale. [...]