The present invention relates to an optical device usable in a head-mounted display which comprises a light conducting element and at least one wafer, and to a head-mounted device which is configured to provide an informative display of augmented reality for a wearer of the head-mounted device and which comprises such an optical device. This optical device (1) comprises: a light conducting element (4) which has two first and second opposite main faces (4a and 4b) and which inputs and conducts by total internal reflections a light (6) received from the light source (5) and partially outputs it out of said element, at least one wafer (2, 3) which comprises an internal surface (2a, 3a) facing a said first main face of said light conducting element, and interposition means in contact with said first main face and said internal surface and defining a gap (11a, 11b) optically isolating said element. According to the invention, said interposition means comprise a peripheral sealing arrangement (10a, 10b) which extends along its peripheral length between and in sealing contact with a peripheral area (2aa, 3aa) of said internal surface and a facing zone (4aa, 4ba) of said first main face, at least one portion of said sealing arrangement exhibiting a locally reduced optical coupling with said first main face to preserve said internal reflections at said at least one portion toward the inside of said element.