Alzheimer’s disease (AD) and vascular dementia (VD) are the two most common types of cognitive impairment. However, there is still a lack of effective tools for their differential diagnosis. In this work, we demonstrated that OEF is differentially (in opposite direction) affected by AD and vascular pathology. Among patients with cognitive impairment, patients with low OEF are associated with more AD pathology and less vascular pathology; and the opposite can be said for patients with high OEF. These findings suggest that OEF may provide valuable information in differentiating AD and VD.