Background: OSA is a common condition that affects at least 4% of males & 2% of female. OSA is primarily characterized by intermittent and recurrent pauses in respiration results in multiple cycles of hypoxia/ reoxygenation with an increased production of reactive oxygen species (ROS). Excess body weight is a major risk factor for snoring and sleep apnoea, Obese patients with SDB may be disadvantaged by greater oxygen desaturation during apneas and hypopneas and by increased event frequency.Aim of work: is to assess markers of hypoxic stress in obese patients with & without OSA & to assess effect of CPAP treatment & effect of weight loss following bariatric surgery upon levels of these markers whenever possible.Subjects & methods: study was performed on 51 obese subjects who had been referred to Chest Department of Kasr Alaini Hospital for clinical suspicion of OSA in order to perform Polysomnography. They were classified into two groups; Cases: consists of 33 obese patients who were diagnosed as obstructive sleep apnea (OSA) & Controls : consists of 18 obese subjects, without OSA as a control group. The two groups were subjected to Full history taking, Clinical examination, Epworth sleepiness scale (ESS) , Anthropometric measurements including body mass index (BMI in kg/m2) waist , hip circumferences, waist/hip ratio and neck circumference, Polysomnographic study, Pulmonary function tests (spirometry), Laboratory examination for estimation of lipid profile (Triglycerides & Total Cholesterol), AST, ALT and FBS, Arterial Blood Gases (ABGs), Serum Leptin, Serum Adiponectin, TBARS, Serum Insulin level measurements by ELISA & Assessment of insulin resistance by calculation of HOMA index.Results: It was found that cases included 10 females (30.3%) & 23 males (69.7%), while controls included 14 females (77.8%) & 4 (22.2%). There was statistically highly significant increase of daytime sleepiness & Epworth sleepiness scale (ESS) among cases compared to controls, also there was statistically significant increase of nocturia among cases compared to controls. As regards the Polysomnographic data, there was statistically highly significant increase in AHI, desaturation Index & duration of desaturation <90%. among cases compared to control subjects. Regarding minimal O2 sat% & average O2 sat% were lower in cases than in the control subjects, and this reduction was statistically significant.also there was statistically significant increase in arrhythmias index among cases than control subjects & there was significant increase in snoring index among cases than control subjects. There was statistically highly significant increase in serum insulin, HOMA index, Leptin & TBARS levels among cases as compared to controls, while there was statistically highly significant reduction in serum adiponectin level among cases in comparison to controls. There was a statistically significant positive correlation between grade of obesity and both serum Cholesterol level & serum TBARS among studied cases. We found that CPAP treatment & bariatric surgery were associated with clinical improvement, improvement Polysomnographic data & serum markers.Conclusion: The studed markers could be used as prognostic factors to assess the response following CPAP treatment or Bariatric surgery.