This observational, cross-sectional, single-center pilot study aims to examine the presence of "Brain Fog" - a cluster of cognitive symptoms impairing memory, attention, and concentration - in patients with severe Obstructive Sleep Apnea Syndrome (OSA). It compares two groups: patients who have been undergoing CPAP treatment for at least 3 months and patients who have not yet begun treatment. Patients with severe OSA (AHI \> 30), evaluated by a pulmonologist for CPAP therapy adaptation or already in therapy for at least 3 months, will be invited to participate in the study at the Sleep Center Clinic. After reviewing and signing informed consent, participants will receive a link to access the Qualtrics platform, where they will complete the online self-report questionnaire under the supervision of trained staff, available to assist if needed. The questionnaire, taking approximately 30 minutes to complete, will be administered once and will collect sociodemographic data (gender, age, marital status, educational level, occupational status, comorbidities, diagnosis date, CPAP adaptation start date, and COVID-19 health status over the past 12 months), including the Brain Fog Scale, currently undergoing Italian validation by our Research Group. Additional measures will include the Epworth Sleepiness Scale (ESS) to assess daytime sleepiness, the Pittsburgh Sleep Quality Index (PSQI) for sleep quality, the Fatigue Severity Scale (FSS), and Pichot's Fatigue Scale for psychophysical fatigue. The study anticipates recruiting 80 outpatients with severe OSA, evenly divided by age and gender into two groups: 40 patients on CPAP for at least 3 months and 40 patients who have not yet started CPAP. As no prior studies have investigated Brain Fog in OSA patients, conventional power calculations could not be applied; hence, 40 patients per group were deemed sufficient to provide preliminary estimates and identify any logistical or methodological issues for larger studies. Statistical analyses will be conducted using Jamovi 2.5.3 software, encompassing descriptive statistics, independent sample T-tests, Pearson's correlation to measure the strength of linear relationships between Brain Fog and other assessment scale scores, and a multiple regression model to examine the effect of independent variables on Brain Fog. In cases of non-normal data distribution, appropriate non-parametric analyses will be applied. The Brain Fog Scale, currently under Italian validation, may affect the reliability and validity of findings. Significant differences in Brain Fog levels are expected between CPAP-treated and untreated patients, with the hypothesis that treated patients will show symptom reduction. CPAP treatment is anticipated to improve memory, attention, concentration, and mental clarity in severe OSA patients. The collected data may also reveal a positive correlation between CPAP use, sleep quality improvement, and reduced Brain Fog symptoms. Furthermore, this study aims to provide preliminary data for calculating the necessary sample size for larger, more rigorous future studies. Clinical implications suggest that CPAP treatment may not only relieve respiratory symptoms of OSA but also offer significant benefits for cognitive functions, thus enhancing the quality of life for patients with severe OSA.