Abstract: The oral microbiome is a complex and highly structured ecosystem composed of diverse microorganisms, including bacteria, fungi, viruses, and other microbes, which establish an intimate symbiotic relationship with the host. Its composition across distinct ecological niches, such as teeth and mucosa, is modulated by multiple factors, including age, genetics, and lifestyle. A stable microbial community acts as an essential barrier for sustaining oral and systemic health. This review systematically examines the structure and function of the oral microbiome under healthy and diseased conditions, with an emphasis on the formation mechanisms of plaque biofilms and their pivotal roles in the initiation and progression of dental caries and periodontitis. Dental caries is predominantly driven by acidogenic and aciduric bacteria, such as Streptococcus mutans and Lactobacillus spp., accompanied by microenvironmental acidification and enamel demineralization. Periodontitis is closely associated with the enrichment of pathogenic microorganisms, including the "red complex" in subgingival plaque, and host immune dysregulation. Furthermore, ecological dysbiosis of the oral microbiome, particularly the abnormal proliferation of pathogens such as Fusobacterium nucleatum and Porphyromonas gingivalis, not only contributes to the development and progression of oral squamous cell carcinoma but also closely correlates with numerous systemic disorders, including cardiovascular diseases, diabetes mellitus, rheumatoid arthritis, and pancreatic cancer, via mechanisms such as inflammatory induction, immunosuppression, and microbial translocation. Systematic elucidation of the ecological characteristics and pathogenic mechanisms of the oral microbiome will provide a critical theoretical foundation for maintaining oral microecological homeostasis and for preventing and treating oral and systemic comorbidities.