Inner membrane component of the type IV (T4S) secretion system that plays a role in surface and host cell adhesion, colonization, biofilm maturation, virulence, and twitching, a form of surface-associated motility. PilN/PilO heterodimers form the foundation of the inner-membrane PilM/PilN/PilO/PilP complex which plays an essential role in the assembly of a functional T4 pilus. Interacts with PilN (PubMed:19857645, PubMed:19857646, PubMed:25917913). Interacts with PilP (via N-terminus) (PubMed:19857645). Interacts with PilM (PubMed:19857645). Deletion mutants lack surface pili.