Operator Mods are an attachment variation available in Call of Duty: Black Ops 4. An operator mod modifies a weapon's behavior in a fundamental way, to a greater degree than regular attachments. Furthermore, operator mods require a special Wildcard (the Primary Operator Mod or Secondary Operator Mod) in order to be equipped.
Not all weapons have operator mods available, but all available operator mods tend to emphasize some aspect of a weapon's design. For example, the Cordite's Belt-Fed operator mod emphasizes the weapon's large magazine capacity by giving it a whopping 600-round magazine.