Abstract Introduction Overweight and obese children are at risk of obstructive sleep apnoea (OSA) and abnormal pulmonary function (PF). Aim Investigate the relationship between body mass index (BMI), OSA on PF in children. Materials & Method Seventy‐four children were recruited. Mixed obstructive apnoea‐hypopnea index (MOAHI), BMI, oxygen saturation (SpO2), forced expiratory volume one second (FEV1), forced vital capacity (FVC) and fractionated exhaled nitric oxide (FeNO) were measured. Results Twenty‐four and thirty children had mild OSA and moderate‐to‐severe OSA respectively. BMI correlated negatively with SpO2 nadir (r = −.363, p = .001). FVC, FEV1 and nadir SpO2 values decreased with OSA severity (p