Oligodendrocytes have received much attention in relation to neurological and psychiatric disorders. The involvement of oligodendrocytes and their myelin in normal brain functions has been suggested by many lines of evidence. The conduction velocity of action potentials along axons is dramatically increased by myelination, that is, the formation of a passive insulator. There is a growing understanding of the functional roles of ion channels and neurotransmitter receptors on oligodendrocytes, and the activity-dependent facilitative effect of oligodendrocytes on conduction velocity has been demonstrated. In this article, we summarize evidence for the ability of oligodendrocytes to monitor neuronal activity and for the facilitation of axonal conduction by oligodendrocytes by mechanisms other than myelination. We suggest the underlying mechanisms for this facilitation in relation to the morphological dynamics of myelinating processes and discuss the physiological roles of the facilitation in information processing.