It is a fraction from zero to one, and it changes with saturation. As water saturation rises in a water-wet rock, water's relative permeability rises and oil's falls. The two curves together decide how a waterflood moves and how much oil is still flowing when the flood reaches a high water cut.
The endpoint, where oil's relative permeability reaches zero, is the residual oil saturation. Laboratory curves depend on wettability and on whether the test is drainage or imbibition, so a curve measured on a cleaned, water-wet plug can misrepresent an oil-wet reservoir.