A Core Object-Oriented (OO) JavaScript developer designs, tests, and implements software solutions using JavaScript's OO features. They use JavaScript's prototypal inheritance and object composition to create reusable code components. They work with OO design patterns like modules, factories, and constructors to build robust and scalable applications. Their responsibilities also include debugging, improving application performance, and ensuring code maintainability. They collaborate with other team members to integrate user-facing elements with server-side logic, prioritize user experience, and ensure the technical feasibility of UI/UX designs. They also keep themselves updated with the latest industry trends to optimize applications.