IMPORTANCE: Oral appliances that move the mandible forward during sleep are suggested as treatment for mild to moderate obstructive sleep apnea. OBJECTIVE: To test whether an adjustable, custom-made oral appliance improves daytime sleepiness and quality of life in patients with daytime sleepiness and snoring or mild to moderate obstructive sleep apnea. DESIGN, SETTING, AND PARTICIPANTS: Ninety-six patients with daytime sleepiness and an apnea-hypopnea index (AHI) lower than 30 were included in a randomized, placebo-controlled, parallel trial in Umeå, Sweden, from May 2007 through August 2011. INTERVENTIONS: Four months' intervention with an oral appliance or a placebo device. MAIN OUTCOMES AND MEASURES: Daytime sleepiness was measured with the Epworth Sleepiness Scale, the Karolinska Sleepiness Scale, and the Oxford Sleep Resistance (OSLER) test. Quality of life was assessed with the Short-Form 36-Item Health Survey (SF-36) and the Functional Outcomes of Sleep Questionnaire (FOSQ). Secondary outcomes included the apnea-hypopnea index, headaches, symptoms of restless legs, and insomnia. RESULTS: Oral appliance therapy was not associated with improvements in daytime sleepiness from baseline to 4-month follow-up when compared with the placebo device; Epworth score >10: 53% at baseline to 24% at follow-up for the oral appliance group vs 54% at baseline to 40% at follow-up for the placebo device group, P = .11; median (IQR) for Karolinska score ≥7/wk: 10 (8 to 14) at baseline to 7 (4 to 9) at follow-up for the oral appliance group vs 12 (6 to 15) at baseline to 8 (5 to 12) at follow-up for the placebo device group, P = .11; mean between-group difference in OSLER test, -2.4 min (95% CI, -6.3 to 1.4). The mean between-group difference for the total FOSQ score was insignificant (-1.2 [95% CI, -2.5 to 0.1]). No domain of the SF-36 differed significantly between the groups. The AHI was below 5 in 49% of patients using the active appliance and in 11% using placebo, with an odds ratio of 7.8 (95% CI, 2.6-23.5) and a number needed to treat of 3. Snoring (P < .001) and symptoms of restless legs (P = .02) were less frequent when using the oral appliance vs placebo, but this did not apply to headache or insomnia. CONCLUSIONS AND RELEVANCE: A custom-made, adjustable oral appliance reduces obstructive sleep apnea, snoring, and possibly restless legs without effects on daytime sleepiness and quality of life among patients with daytime sleepiness and snoring or mild to moderate sleep apnea. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT00477009.