An implant for treating obstructive sleep apnea includes a flexible chamber, a fluid reservoir in fluid communication with the flexible chamber, and a fluid transfer assembly in communication with the fluid reservoir and the flexible chamber for transferring fluid therebetween for selectively modifying the rigidity of the flexible chamber. The flexible chamber is implantable within the soft tissue of an oropharyngeal airway of a patient, such as within the tongue, the soft palate, or the pharyngeal wall. The fluid reservoir and the fluid transfer assembly are implantable within the inframandibular region of the patient. The fluid transfer assembly is selectively engageable by the patient for transferring fluid between the two chambers for modifying the rigidity, flexibility, and/or shape of the flexible chamber with minimal or no change to the volume of the implant.