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 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.