Standardised surveillance data on oral health is lacking in developing nations, hindering the progress towards achieving global oral health. Furthermore, there was no summary of evidence on the relationship between oral hygiene and metabolic syndrome (MetS). The aims of the thesis were: 1) to investigate the prevalence and factors associated with oral hygiene practice among Indonesian adolescents; 2) periodontitis among Indonesian adults; 3) to quantitatively synthesise the body of evidence of the association between oral hygiene and MetS. This thesis consisted of two empirical studies and a systematic review and meta-analysis. The first study used cross-sectional data from the 2015 Indonesia Global School-based Health Survey, covering 11,142 students aged 11-18 years. Around 10.8% of students had less than twice a day tooth-brushing. Some lifestyle and psychosocial factors were positively associated with oral hygiene practice. The second study used cross-sectional data from the 2018 Indonesia National Health Survey, covering 13,359 dentate individuals aged 35+ years. Over 40% of adults had periodontitis. While the relationship between MetS and periodontitis was not shown, hyperglycaemia was positively associated with periodontitis. The third study was a systematic review and meta-analysis. A search on PubMed and Web of Science databases until March 17th, 2021, was undertaken to identify eligible studies on the relationship between oral hygiene and MetS. We found overall negative relationships between oral hygiene status, the frequency of tooth brushing, interdental cleaning and MetS. Nevertheless, substantial heterogeneity for the exposure of tooth-brushing frequency and inconsistent findings for oral hygiene status in subgroup analyses were noted. There was inadequate evidence on the relationship between dental visits and MetS. Overall, our empirical studies indicated a need to improve oral health in Indonesia. Integrating oral health promotion into general health actions addressing adolescents' families, schools, and social environments is warranted to promote healthier behaviours. Integrating oral health into routine diabetes care might also reduce the disease burden. Regarding MetS, further studies examining its role in periodontitis would be worthwhile. Similarly, high-quality longitudinal studies are needed to confirm the relationship between oral hygiene and MetS.