The objective of the study was to determine the effects of nasal continuous positive airway pressure (nCPAP) therapy on left ventricular (LV) function and electrocardiographic parameters in newly diagnosed moderate/severe obstructive sleep apnea (OSA) patients without cardiovascular comorbidities and medical treatments. We examined 44 patients who underwent overnight polysomnography together with 24-hour Holter electrocardiography, cardiopulmonary exercise testing including heart rate recovery at 1 minute (HRR-1), echocardiography, surface electrocardiography, and those who were diagnosed with moderate/severe OSA apnea—hypopnea index ≥ 15. After 3 months of nCPAP treatment, the above-mentioned examinations were repeated. Forty-four patients completed the treatment period. Twelve weeks on effective nCPAP induced a significant increase in the mitral E/A ratio (P = 0.001), as well as reductions in isovolumic relaxation time (P = 0.001) and mitral deceleration time (DT) (P = 0.002). There were no significant differences in LV ejection fraction, LV mass index, and pulsed wave Doppler parameters. Mean heart rate was 79.2 ± 12.5 pulses/minute, maximum P-wave duration 117.5 ± 8.6 msec, P-wave dispersion (PWd) 54.6 ± 10.2 msec, corrected QT interval (QTc) 436.5 ± 40.5 msec, and QT dispersion (QTd) 46.3 ± 7.1 msec, which significantly decreased to 70.4 ± 9.6 pulses/minute (P < 0.001), 111.5 ± 8.7 msec (P < 0.001), 51.6 ± 8.9 msec (P < 0.001), 418.4 ± 31.2 msec (P < 0.001), and 33.8 ± 3.4 msec (P < 0.001), respectively. Exercise capacity at baseline determined as 10.5 ± 2.2 metabolic equivalents (METS) and HRR-1 (20.6 ± 11.7 bpm) significantly increased (12.1 ±1.5 METS and 27.4 ± 8.6 bpm). There was no significant difference in aortic root parameters. Three-month nCPAP therapy significantly increased LV shortening fraction, with no effect on systolic function or aortic root diameters and a positive effect on heart rate, PWd, HRR-1, QTc and QTd time following nCPAP therapy.