The oral microbiota, comprising over 700 bacterial species, plays a critical role not only in oral health but also in systemic diseases, including gastrointestinal (GI) cancers. Increasing evidence demonstrates that oral dysbiosis contributes to the initiation and progression of colorectal, gastric, esophageal, and pancreatic cancers through mechanisms involving chronic inflammation, immune modulation, carcinogenic metabolite production, and microbial translocation along the oral–gut axis. Key pathogens such as Fusobacterium nucleatum and Porphyromonas gingivalis have been detected within GI tumor tissues, where they influence tumor microenvironment remodeling, metastatic potential, and treatment resistance. Clinically, distinct oral microbial signatures offer promising non-invasive biomarkers for early cancer detection and prognosis. Preventive strategies targeting oral hygiene and periodontal health, alongside therapeutic approaches modulating the oral microbiome, hold substantial potential for reducing GI cancer risk. Understanding oral–GI microbial interactions through standardized and multi-omic methodologies will further advance precision oncology.