Background Individual reactions to traumatic stress vary dramatically, yet the biological basis of this variation remains poorly understood. Recent studies have demonstrated surprising plasticity of oligodendrocytes and myelin in the adult brain, providing a potential mechanism by which aberrant structural and functional changes arise in the brain following trauma exposure. Methods We tested the hypothesis that gray matter myelin contributes to traumatic stress-induced behavioral variation. We exposed adult rats to a single, severe stressor and used a multimodal approach to characterize avoidance, startle, and fear-learning behavior. We quantified oligodendrocyte and myelin content in multiple brain areas and compared these measures to behavioral metrics. We then induced overexpression of the oligodendrogenic transcription factor Olig1 in the adult rat dentate gyrus (DG) to test the potential, causal role of oligodendrogenesis in behavioral variation. Lastly, T1-/T2-weighted estimates of myelin were compared to trauma-induced symptom profiles in humans. Results Oligodendrocytes and myelin in the DG of the hippocampus positively correlated with stress-induced avoidance behaviors in male rats. In contrast, myelin levels in the amygdala positively correlated with contextual fear learning. Olig1 overexpression increased place avoidance compared to control virus animals, indicating that increased oligodendrocyte drive in the DG is sufficient to induce an avoidance behavioral phenotype. Finally, variation in myelin correlated with trauma-induced symptom profiles in humans in a region-specific manner that mirrored our rodent findings. Conclusions These results demonstrate a species-independent relationship between region-specific, gray matter oligodendrocytes and myelin and differential behavioral phenotypes following traumatic stress exposure. This study provides a novel biological framework for understanding the mechanisms that underlie individual variance in sensitivity to traumatic stress.