Introduction: Oral health is an essential component of children's overall health, growth, nutrition, learning ability, and quality of life. Establishing appropriate toothbrushing habits during early childhood requires both effective oral health education and a supportive school environment. This study aimed to assess oral health knowledge among second-grade primary school students and examine its relationship with dental caries prevalence and the availability of school toothbrushing facilities. Goal: To evaluate oral health knowledge among second-grade students in selected public primary schools in Ulaanbaatar and determine its association with dental caries prevalence, sex, and the school toothbrushing environment. Materials and Methods: A school-based cross-sectional analytical study was conducted in May 2026 among 971 second-grade students from 12 public primary schools in Khan-Uul, Chingeltei, and Bayanzurkh districts of Ulaanbaatar. Schools with and without designated toothbrushing facilities were selected using a 1:2 ratio. Oral health knowledge was assessed using a structured questionnaire consisting of 10 questions covering daily oral hygiene, toothbrushing practices, dietary factors, and preventive care. Dental caries were assessed through clinical oral examinations. Data were analyzed using descriptive statistics, the Chi-square test, and Odds Ratios (ORs) with 95% confidence intervals. The study was conducted with the approval of the Research Ethics Committee of the Mongolian National University of Medical Sciences (MNUMS), under Approval No. 26/03, issued in 2026. Results: Overall, 74.77% of students had dental caries, indicating a high prevalence. No statistically significant difference in caries prevalence was observed between boys and girls. Students attending schools equipped with toothbrushing facilities had a significantly lower prevalence of dental caries than those attending schools without such facilities (OR = 0.57; 95% CI: 0.42 0.76). Regarding oral health knowledge, 59.84% of students demonstrated poor knowledge, 37.88% moderate knowledge, and only 2.88% good knowledge. Students showed relatively good knowledge of cariogenic foods and recommended toothbrush replacement intervals; however, knowledge of proper toothbrushing techniques, appropriate toothbrush selection, and the importance of preventive dental visits remained insufficient. Conclusion Most second-grade students had inadequate oral health knowledge and a high prevalence of dental caries. School toothbrushing facilities were associated with a lower risk of dental caries. Strengthening practical school-based oral health education, establishing standardized toothbrushing facilities, and integrating regular preventive dental examinations into school health programs may contribute to improving children's oral health and reducing dental caries.