Introduction: The distribution and severity of dental caries among preschool children vary according to the socio-economic and ethnic differences within and between countries. Understanding socio-economic influences on child oral health could inform early interventions to reduce the oral health burden throughout the life-cycle. The aim of this study is to examine the socio-economic and ethnic influences on oral health among preschoolers in Kegalle, Sri Lanka. Methods: The study involved 784 children aged between 48--72 months recruited from 84 pre-schools in the Kegalle district in Sri Lanka. Cross-sectional data were collected by means of an oral examination of the children and a self-administered questionnaire to their parents/caregivers. The Early Childhood Oral Health Impact Scale (ECOHIS) was used to assess Oral Health related Quality of Life (OHQoL). Univariate and multivariate models of Poisson regression were used to investigate the associations between the variables. Results: Compared to children whose fathers had tertiary education, those whose fathers did not study beyond grade 5, had more caries measured in terms of decayed, missing and filled surfaces (dmfs) (IRR = 2.30; 95 CI: 1.30, 4.06; p <0.01) and experienced poor OHQoL at child (IRR = 2.52; 95 CI: 1.20, 5.31; p <0.05) and family (IRR = 1.59; 95 CI: 1.11, 2.27; p <0.05) levels. However, lower educational attainment among mothers was associated with better OHQoL among children. Compared to the Sinhalese ethnic group, Tamils had more gingival bleeding (bleeding surfaces) (IRR = 3.04; 95 CI: 1.92, 4.81; p <0.001) and poor OHQoL at child level (IRR = 2.07; 95 CI: 1.19, 3.60; p <0.01), whereas Muslims had poor OHQoL at family level (IRR = 1.42; 95 CI: 1.10, 1.84; p <0.01). Children of low-income families had more gum bleeding (IRR = 1.00; 95 CI: 0.99, 1.00; p <0.05) compared to children of high-income families. Conclusions: Socio-economic and ethnic differences in oral health outcomes exist among this population of preschoolers. Interventions targeting children of fathers with low educational levels and ethnic minority groups are required to reduce inequalities in oral health in Sri Lanka and other similar countries.