Obesity hypoventilation syndrome (OHS) is a condition in which severely overweight people fail to breathe rapidly or deeply enough, resulting in low oxygen levels and high blood carbon dioxide (CO₂) levels. The syndrome is often associated with obstructive sleep apnea (OSA), which causes periods of absent or reduced breathing in sleep, resulting in many partial awakenings during the night and sleepiness during the day. The disease puts strain on the heart, which may lead to heart failure and leg swelling.

Obesity hypoventilation syndrome is defined as the combination of obesity and an increased blood carbon dioxide level during the day that is not attributable to another cause of excessively slow or shallow breathing.

The most effective treatment is weight loss, but this may require bariatric surgery to achieve. Weight loss of 25 to 30% is usually required to resolve the disorder. The other first-line treatment is non-invasive positive airway pressure (PAP), usually in the form of continuous positive airway pressure (CPAP) at night. The disease was known initially in the 1950s, as "Pickwickian syndrome" in reference to a Dickensian character.