Usually in bogs, where its rhizomatous habit makes it a mat-former locally rivaling in importance C. lasiocarpa, with which it is rather frequently confused when vegetative; relic in older coniferous swamps and also found on wet lake shores, restricted to acidic soils.

The narrow, involute leaves are similar to those of Carex lasiocarpa, but less wiry, the basal sheaths are much spongier than the hard ones of C. lasiocarpa, and the ventral surface of the lower leaf sheaths do not tear to form a ladder-like arrangement of fibers.Carex oligosperma is restricted to acidic soils while C. lasiocarpa occurs in calcareous as well as acidic settings.