The short answer is that the oral appliance used while sleeping may help some symptoms of sleep apnea, but not others.
An oral appliance that helps push the lower jaw forward to make more room in the back of the mouth has NOT been shown to improve daytime sleepiness or other measures of a good night’s sleep. The oral appliance DOES decrease apnea scores, snoring and restless legs. These improvements may be valuable for bed partners who sleep each night enduring the breathing irregularities and lower limb restlessness.
Even after 4 months of treatment the oral apppliance did not improve the patient’s measures of sleepiness, sleep resistance quality of life or functional outcomes of sleep.
However, I imagine that the bed partners slept better. (Photo above inserted for cuteness only. No appliance used in this little patient)