<p>Emerging evidence indicates that oral microbes are closely related to gastric microbes and gastric lesions, including gastric atrophy, intestinal metaplasia and gastric cancer (GC). <i>Helicobacter pylori</i> is a key pathogen involved in GC. However, the increasing prevalence of <i>H. pylori</i>-negative GC and gastric dysbiosis in GC patients emphasize the potential role of other microbial factors. In this review, we discussed the current evidence about the relationship between the oral–gastric microbial axis and oral and gastric health. Epidemiologic evidence indicates that poor oral hygiene is related to greater GC risk. Multiple oral-associated microbes are enriched in the stomach of GC patients. Once colonizing the stomach, oral-associated microbes <i>Streptococcus anginosus</i> and <i>Prevotella melaninogenica</i>, are involved in gastric inflammation or carcinogenesis. Microbial metabolites such as lactate, nitrite, and acetaldehyde promote malignant transformation. The stomach, as a checkpoint of microbial transmission in the digestive tract, is of great importance since the link between oral microbes and intestinal diseases has been emphasized. Still, new technologies and standardized metrics are necessary to identify potential pathogenetic microbes for GC and the core microbiota, interactions, richness, colonization, location and effect (CIRCLE). In the future, oral microbes could be candidates for noninvasive indicators to predict gastric diseases.</p>