Plasminogen activator inhibitor-1 (PAI-1) has a significant role in fibrinolysis, atherogenesis, cellular senescence, and chronic inflammation. OSA (obstructive sleep apnea) leads to increased PAI-1 levels and the development of cardiovascular disease (CVD). The aim of this study was to determine the effects of CPAP therapy in patients with severe OSA on coagulation parameters and PAI-1. This prospective, controlled study enrolled 57 patients who were newly diagnosed with severe OSA, 37 of whom had had a good CPAP adherence after 6 months of therapy (usage of the device for at least 4 h per night), and their data were analyzed. The analysis showed a statistically significant increase in D-dimer values (415 (316.5-537.5) before CPAP therapy vs. 499 (327-652) after therapy, p = 0.0282) and a decrease in fibrinogen values (3.665 ± 0.752 before CPAP therapy vs. 3.365 ± 0.771) after therapy, p = 0.0075)). The PAI-1 concentration values before and after CPAP therapy did not differ significantly (17.35±7.01 ng/ml before CPAP therapy vs. 17.42±6.99 ng/ml after therapy, p = 0.9367). This study shows a tendency to improve the fibrinolytic capacity in patients with OSA after CPAP therapy although PAI-1 levels did not differ significantly.