Background: Obstructive sleep apnea (OSA) which is a very common form of sleep-disorder with associated health risks has been associated with much more insidious conditions, like hypertension, heart disease, diabetes, stroke and even daytime somnolence. But we have not enough research-based information regarding the co-relation between BMI and severity of obstructive sleep apnea. Aim of the Study: The aim of this study was to assess the co-relation between BMI and severity of obstructive sleep apnea. Methods: This was an observational study which was conducted in the Department of Otolaryngology &, Head Neck Surgery, Bashundara Ad-Din Medical College and Hospital, Bangladesh during the period from January 2121 to December 2021. A total of 57 patients with obstructive sleep apnea (OSA) were included as the subjects. The ethical committee of the mentioned hospital approved this study. All demographic data along with BMI and the severity of OSA of the participants were recorded and analyzed. Data were processed and analyzed by using MS Office and SPSS version 23 programs as per need. Results: In this study, in analyzing the correlation between BMI and severity of OSA as per patient’s diagnosis and titration based on two nights’ study, we observed that, there was not any significant correlation of BMI of the patients with mild, moderate or even severe stage OSA. In comparing the mild, moderate and severe cases of OSA with their mean BMI values 30.66±6.23, 28.47±6.78 and 30.43±4.32 and the P values were found as 0.755, 0.554 and 0.454 respectively. Conclusion: According to the findings of this study we can conclude that, there is not any significant correlation of patient’s BMI with their mild, moderate or even severe conditions of obstructive sleep apnea.