An IBM z/OS developer primarily works on IBM's z/OS operating system used for mainframe computers. They design, develop, and implement software applications, ensuring they run effectively on this system. Key tasks include system installation and customization, diagnosing and resolving system issues, and performance tuning. They also manage data storage and security, and support other software like databases and middleware. They often work in a team, collaborating with other IT professionals like system administrators, network engineers, and security analysts. Their work is crucial for businesses that rely on mainframe computers for large-scale, critical applications like transaction processing and enterprise resource planning.