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For decades, cancer risk has been explained mainly by local factors. However, emerging evidence shows that the oral microbiome acts as a systemic modifier of oncogenesis well beyond the head and neck. This review synthesizes clinical and mechanistic data linking dysbiotic oral communities, especially Porphyromonas gingivalis, Fusobacterium nucleatum, and Treponema denticola, to malignancies across gastrointestinal, respiratory, hepatobiliary, pancreatic, breast, and urogenital systems. We summarize organ-specific associations from saliva, tissue, and stool studies, noting the recurrent enrichment of oral taxa in tumor and peri-tumoral niches of oral, esophageal, gastric, colorectal, lung, pancreatic, liver, bladder, cervical, and breast cancers. Convergent mechanisms include the following: (i) persistent inflammation (lypopolysacharide, gingipains, cytolysins, and collagenases); (ii) direct genotoxicity (acetaldehyde, nitrosation, and CDT); (iii) immune evasion/suppression (TLR/NLR signaling, MDSC recruitment, TAN/TAM polarization, and TIGIT/CEACAM1 checkpoints); and (iv) epigenetic/signaling rewiring (NF-κB, MAPK/ERK, PI3K/AKT, JAK/STAT, WNT/β-catenin, Notch, COX-2, and CpG hypermethylation). Plausible dissemination along an oral-gut-systemic axis, hematogenous, lymphatic, microaspiration, and direct mucosal transfer enables distal effects. While causality is not yet definitive, cumulative data support oral dysbiosis as a clinically relevant cofactor, motivating biomarker-based risk stratification, saliva/stool assays for early detection, and microbiome-targeted interventions (periodontal care, antimicrobials, probiotics, and microbiota modulation) alongside conventional cancer control.