Embodiments of the disclosure describe an on-head detection technique for an HMD that includes an optical sensor positioned to detect light reflected from at least one of a face of a user or a lens worn by the user. A flexible frame assembly supports an image source and further supports the optical sensor relative to the face or the lens when worn by the user. The flexible frame assembly flexes such that the optical sensor moves closer to at least one of the face or the lens when the HMD is worn by the user. Embodiments determine whether the user is wearing the HMD based on optical sensor data output from the optical sensor.