Despite the dramatic decline in the incidence of common opportunistic infections (OIs) after antiretroviral therapy initiation, they remain a significant cause of morbidity and mortality among children with HIV. For better interventions, information regarding the incidence and predictors of common OIs is essential for Children living with HIV. Still, there is a lack of studies in low and middle-income countries, including Ethiopia. Therefore, this study aims to assess the incidence and predictors of common OIs among Children living with HIV on anti-retroviral therapy (ART) at public health institutions in Bahir Dar City, Northwest Ethiopia. The reasons for excluding children not on ART is we want to study the effectiveness of chronic HIV care service, ART drugs and OIs prophylaxis drugs for the prevention of common OIs because it is obvious that the occurrence of OIs in children not on ART and OIs prophylaxis drugs is high. A health institution-based retrospective cohort study was done among 403 Human Immunodeficiency Virus-infected children at public health institutions in Bahir Dar City from 2010 to 2020. Data was entered using Epi-data version 4.6 and analyzed using STATA 14.0. A bivariable Cox-proportional hazards regression model was employed to appreciate the relationship between each explanatory variable with the outcome variable. In the bivariable analysis, variables with a p-value of less than 0.25 were candidates for the multivariable proportional hazard model. The Cox proportional hazards model was used to determine predictors of common opportunistic infections at a 5% significance level. The overall incidence rate of common opportunistic infections was 7.06 with a 95% confidence interval ((CI) 5.78, 9.75) per 100 person-years of observation. Statically significant predictors were World Health Organization (WHO) clinical stage III and IV (adjusted hazard ratio (AHR) = 1.90; (95% CI 1.34, 2.75), having fair/poor adherence to anti-retroviral therapy (ART) (AHR) = 1.80; (95% CI 1.25, 2.94) and hemoglobin level < 10 g/dl (AHR) = 2.00; (95% CI 1.36, 2.89). The overall incidence rate of common OIs among children living with HIV on ART was high. Independent predictors of common OIs among children on ART were advanced-stage of HIV disease, poor ART adherence, and lower hemoglobin level. Therefore, we recommend strongly working on the prevention of advanced stages of HIV disease and improving poor ART adherence to prevent the incidence of OIs among children living with HIV on ART.