There are more than 80 different oral devices on the market for the treatment of obstructive sleep apnea and snoring, according the dental sleep medicine web site "Snoring Isn’t Sexy." The great preponderance of them are varieties of “mandibular repositioning dental appliances,” devices that move the lower jaw forward. Most of the remainder are “tongue retaining appliances.” Both serve to reduce the likelihood of the sleeper’s tongue falling backward far enough to block the airway.
Oral appliances are most effective in the treatment of mild to moderate sleep apnea although they do provide a treatment alternative for patients with severe OSA who cannot or will not tolerate positive airway pressure therapy. Despite their double-retainer-like appearance, oral appliances are said to be comfortable to use. Sometimes for more complicated sleep apnea an oral appliance and CPAP are used in combination.
In the United States, oral devices to treat OSA cannot be sold over the counter. They must be prescribed by a physician and fitted by a dentist, oral surgeon or ENT who has sleep medicine experience. The American Academy of Dental Sleep Medicine (AADSM) provides a search tool to locate a dentist near you who is so qualified. The site also has much other useful information. The AADSM also accredits dental facilities for sleep medicine - here is the link.
Wikipedia is also helpful.