Nasal CPAP will improve cardiopulmonary exercise test (CPET) performance in patients with overlap syndrome(COPD and OSA). Nasal CPAP is proven to improve cardiopulmonary exercise testing in patients with OSA. The investigators hypothesis is that patients with overlap syndrome will have a greater improvement in their cardiopulmonary exercise testing besides a possible improvement in their lung function test and airway resistance.

Patients presenting to the Oklahoma City VA sleep clinic with obstructive sleep apnea syndrome and a desire to initiate CPAP treatment will be included if they have overlap syndrome. Patients will undergo a spirometry and CPET before being started on CPAP as part of standard clinical care. The CPET will be repeated after 8-12 weeks of CPAP use. Patients will be excluded if they have any contraindication to mild exercise or they cannot perform an exercise test due to limited mobility.

Inclusion Criteria:

• Obstructive sleep apnea (OSA)
• COPD

Exclusion Criteria:

• Inability to perform exercise test due to musculoskeletal limitation
• Cardiac condition preventing patient from mild exercise
• Continuous oxygen supplement

Patients with OSA and COPD will perform CPET before and after use of CPAP.