Rising on the plane of Elysium is a great river, broad in its banks, whose waters flow smoothly across the upper realms. Its surface is calm, barely marred by ripples, and its depths are deeper than any line can fathom. The water isn't dangerous, but sweet - indeed fragrant - to those who stand on its shores. Its surface is calm, yet it has its share of dangerous currents and creatures in the depths below. This is the River Oceanus.
Like its sister the Styx, the Oceanus is a worldspanning river, a waterway between realms, layers, and planes. Its headwaters lie in Thalasia, on the plane of Elysium. From there it flows through all of Elysium's layers and eventually breaks through the boundary between the planes, passing through the top layer of the Beastlands. After this, it crosses another boundary and enters Arborea, passes through the first layer, and finally disappears somewhere in that plane's second layer, Ossa.
Although Oceanus looks like a normal river, it's far from that. In some places the river is, but in other places it isn't. It flows in and out of existence without once breaking its continuous flow, defying the logic of connection. A ship might sail through one plane or layer for an hour or a week (depending upon the DM's plans). Passage through the barriers can be arduous or instantaneous. Sometimes, a body just slips from one plane to the next while he wasn't looking.
Oceanus is a commonly used path between planes and the layers of planes. Trading vessels sail up and down its length, and small towns line its banks. Travelers can usually find a boat to hire somewhere along its shores.
Note: The River Oceanus originally appeared in this form in the Planescape AD&D campaign setting from TSR. Its use here is for the purposes of providing context for the campaign only.