Oral appliances, and systems and methods, which include oral appliances are described herein. An example oral appliance device for treatment of a sleep disorder or condition in a subject can include an upper portion configured to receive at least one maxillary tooth of the subject, and a lower portion configured to receive at least one mandibular tooth of the subject. The upper and lower portions are engageable such that the portions maintain a restricted orientation relative to each other when engaged, and the engaged restricted orientation is configured to maintain the mandible of the subject in a predetermined clinically beneficial orientation relative to the maxilla.
