Despite increasing scientific interest in the effectiveness of mandibular advancement device (MAD) for the treatment of obstructive sleep apnea (OSA), laypeople lack knowledge about this treatment option. To investigate content, quality and readability of the online information regarding MAD. Google, Yahoo and Bing were searched for "sleep apnea", "mandibular advancement device" and "oral appliance". Websites were analyzed for content (multidisciplinary care team, qualified dentist, treatment contraindications and side effects), as well as for quality (DISCERN instrument, HONcode) and readability scores (Flesch Reading Ease, FRE, and Flesch-Kincaid Reading Grade, FKG). 155 websites were included: 53% from health professionals, 20% commercial, 17% academic, 10% from non-health professionals. Content was incomplete, especially for commercial ones. 71.61% websites failed to acknowledge treatment contraindications, approximately 40.00% did not mention side effects and the need for a multidisciplinary care team, while 22.58% did not address the need to consult a qualified dentist. Quality and reliability were poor. Mean DISCERN score was 39.93 (95% CI 37.90-41.96), with lower scores for commercial websites compared with others. Only nine websites displayed HONcode certification. Readability was quite difficult, with mean FRE score of 59.50 (95% CI 57.58-61.42) and mean FKG level of 6.92 (95% CI 6.64-7.21). Health care professionals should be aware that currently available online information do not fulfill the most important aspects of MAD therapy and may be difficult to understand by laypeople. This could contribute to cause delays in appropriate OSA care and unrealistic treatment expectations, increasing the risk of treatment discontinuation.