<p>Current concepts suggest a close relationship between dysfunction of neuronal circuits, abnormalities of brain myelination, and alterations in the structure and function of oligodendrocyte lineage cells. Previously, we reported a significant decrease in the numerical density (Nv) of oligodendrocytes (Ol) and oligodendrocyte clusters (OlC) in layer 5 of the dorsal anterior cingulate cortex (dACC; Brodmann area 24, BA24) in schizophrenia; however, the NvOl and NvOlC in layer 3 have not yet been investigated. Reduced connectivity between the ACC and the attention, salience, and default mode networks is characteristic of schizophrenia and is thought to underlie impaired control of affective and cognitive aspects of goal-directed behavior. These interactions are maintained through extensive cortico-cortical connectivity, with a potentially strong contribution from layer 3 pyramidal neurons. We used the optical disector method to estimate the numerical density of oligodendrocytes and oligodendrocyte clusters in layer 3 of the dACC in schizophrenia (18 cases) compared with healthy controls (13 cases). A significant reduction in the numerical density of oligodendrocytes (− 28%; <i>p</i> &lt; 0.001) and oligodendrocyte clusters (− 29%; <i>p</i> = 0.012) was found in schizophrenia compared with controls. Strong positive correlations between NvOl and NvOlC were observed in both the control group (<i>R</i> = 0.93; <i>p</i> &lt; 0.001) and in the schizophrenia group (<i>R</i> = 0.79; <i>p</i> &lt; 0.001). These findings suggest that the deficits of oligodendrocytes and oligodendrocyte clusters may contribute to altered functional connectivity in the networks involving the dACC, a key cortical hub implicated in schizophrenia.</p>