This study aims to measure oral health knowledge and practice level among dental clinic patients, and examine their relationships with decayed, missing and filled teeth (DMF) index and the oral health-related quality of life (QOL). Empirical data have been collected from 311 patients through structured questionnaires and dental examinations conducted in Busan area. According to the analysis of causal relationship, oral health knowledge turned out to have a direct effect on oral health practices, which exerted both direct and indirect effects on DMF index and oral health-related QOL. While DMF index directly affected the oral health-related QOL, oral health knowledge proved to have only an indirect effect on DMF index and oral health-related QOL, mediated by oral health practices. Considering all these findings, effective oral health education and other intervention programs should focus on motivating people to participate in the active practices of desirable oral health behaviors, which will lead to the prevention of oral diseases and the subsequent improvement in the QOL.