The Optical Camouflage is a type of technology, created by Verde, that allows the users to camouflage themselves. Since Verde was afraid his subordinates would eventually try and betray him, he made it so that only people under a certain age could see people camouflaged by it.
These were used by Verde's Optical Camouflage Troop, Parenza and Tras, to attempt to assassinate Mafia Leaders and their successors, which included Tsuna.