Obstructive sleep apnea syndrome (OSAS) is a common form of sleep-disordered breathing (SDB) characterized by repetitive episodes of cessation of breathing during sleep due to upper airway collapse. It causes sleep fragmentation, disabling daytime sleepiness, impaired cognitive function and poor quality of life. In addition, OSAS is associated with non-fatal and fatal cardiovascular consequences including sudden death, in addition to an increased risk of road traffic accidents. Continuous positive airway pressure (CPAP) is considered as the first-line treatment for OSA. Oral appliance has been shown to reduce the severity of sleep disordered breathing and leads to symptomatic improvement especially in mild to moderate OSA. The compliance with CPAP is low particularly in mild or moderate OSA patients and it is not a curative treatment of OSA. It has to be used in every night on a regular basis. Weight reduction has always been advocated in patients with OSA who are overweight and may lead to improvement in the severity of OSA. The existing studies about weight loss are limited by small sample size, short duration (<6 months), focus on very low calorie diet program or surgically induced weight loss program only. However, none of them have applied lifestyle modification program (LMP) which emphasizes on long term lifestyle and behavior change. Therefore, the investigators plan to conduct a randomized controlled trial among Chinese OSA patients by comparing the efficacy of LMP against usual clinical lifestyle advice alone on the improvement of OSA symptoms.