Oligodendrocytes are in contact with neurons, wrap axons with a myelin sheath that protects their structural integrity, and facilitate nerve conduction. Oligodendrocytes also form a syncytium with astrocytes which interacts with neurons, promoting reciprocal survival mediated by activity and by molecules involved in energy metabolism and trophism. Therefore, oligodendrocytes are key elements in the normal functioning of the central nervous system. Oligodendrocytes are affected following different insults to the central nervous system including ischemia, traumatism, and inflammation. The term oligodendrogliopathy highlights the prominent role of altered oligodendrocytes in the pathogenesis of certain neurological diseases, not only in demyelinating diseases and most leukodystrophies, but also in aging and age-related neurodegenerative diseases with abnormal protein aggregates. Most of these diseases are characterized by the presence of abnormal protein deposits, forming characteristic and specific inclusions in neurons and astrocytes but also in oligodendrocytes, thus signaling their involvement in the disease. Emerging evidence suggests that such deposits in oligodendrocytes are not mere bystanders but rather are associated with functional alterations. Moreover, operative modifications in oligodendrocytes are also detected in the absence of oligodendroglial inclusions in certain diseases. The present review focuses first on general aspects of oligodendrocytes and precursors, and their development and functions, and then introduces and updates alterations and dysfunction of oligodendrocytes in selected neurodegenerative diseases with abnormal protein aggregates such as multiple system atrophy, Lewy body diseases, tauopathies, Alzheimer’s disease, amyotrophic lateral sclerosis, frontotemporal lobar degeneration with TDP-43 inclusions (TDP-43 proteinopathies), and Creutzfeldt-Jakob's disease as a prototypical human prionopathy.