A titratable thermoplastic mandibular advancement device (MAD) is clearly an effective treatment option in some patients with obstructive sleep apnea (OSA). Determining which patients may be more likely to respond to treatment with thermoplastic MADs and to adhere to treatment would be of obvious clinical relevance. This was an experimental descriptive study (N = 60). Patients with OSA were instructed to wear a titratable thermoplastic MAD for 3 months. Treatment success was defined as a ≥50% reduction from baseline in the apnea-hypopnea index (AHI) or AHI <10 when wearing MAD. Adherence was defined as MAD use ≥5 nights/week. Treatment was successful in 66.7% of patients and 60.0% were adherent. All polysomnographic parameters and visual analogue scale scores (sleep quality, snoring, waking refreshed) were significantly improved after treatment. The patients in whom treatment failed had significantly higher neck circumferences (39.3 cm vs. 37.5 cm, p = 0.014), higher baseline AHI values (26.6 vs. 18.0, p = 0.016), and smaller AHI reduction (-31.8 vs -53.1, p < 0.001) than those in the group in whom treatment succeeded. There were no significant differences in polysomnographic, cephalometric, or visual analogue scale measures between patients for whom treatment was and was not successful, regardless of baseline values or the change rates after the MAD was placed. Titratable thermoplastic MADs can improve indicators of sleep quality, even in patients in whom treatment is considered to have failed.