O-GlcNAcase inhibitors (OGAi) have emerged as a promising therapeutic strategy in Alzheimer’s disease (AD) by enhancing O-GlcNAcylation, which competes with tau phosphorylation and reduces tau aggregation. However, the Phase II clinical trial failure of ceperognastat, marked by accelerated cognitive decline in the treatment group, has raised significant safety concerns. Here, we examined the acute synaptic effects of three structurally distinct OGAi compounds—ceperognastat, ASN90, and MK8719—in mouse hippocampal slices. Electrophysiological recordings revealed suppression of both short- and long-term synaptic plasticity, including paired-pulse facilitation/depression and long-term potentiation. Immunohistochemical analysis confirmed disrupted synaptic protein levels (increased PSD-95, reduced Synaptophysin 1) and a biphasic shift in tau phosphorylation. These convergent findings suggest a class-wide synaptotoxic mechanism and call for a great caution in the development of disease-modifying therapies in AD. We argue that preclinical drug screening for synaptic functionality is essential in CNS-targeted therapeutic pipelines.