The oral cavity presents a highly diverse microbial composition. All the three domains of life, Bacteria, Eukarya, and Archaea, as well as viruses constitute the oral microbiome. Bacteria are among the most abundant microorganisms in the oral cavity, followed by viruses, fungi, and Archaea. These microorganisms tend to live in harmony with each other and with the host by preventing the colonization of oral sites by exogenous microorganisms. Interactions between the host and its microbiota are crucial for keeping ecological stability in the oral cavity and a condition compatible with oral health. This chapter focuses on describing the oral microbiota in healthy individuals based on both targeted and nontargeted genome sequencing methods and the functional activity played by those microorganisms based on metagenomic, metatranscriptomic, metaproteomic, and metabolomic analyses. Additionally, this chapter explores mutualistic and antagonistic microbe-microbe relationships. These interactions are mediated by complex mechanisms like cross-feeding networks, production of bacteriocins and secondary metabolites, synthesis of pH-buffering compounds, and the use of universal signaling molecules. At last, the role played by host-microbe interactions on colonization resistance and immune tolerance will help provide a better understanding about the harmonious and peaceful coexistence among host and microbial cells under oral health-related conditions.