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The monolithic server model where a server is the unit of deployment, operation, and failure is meeting its limits in the face of several recent hardware and application trends. To improve heterogeneity, elasticity, resource utilization, and failure handling in datacenters, we believe that datacenters should break monolithic servers into disaggregated, network-attached hardware components. Despite the promising benefits of hardware resource disaggregation, no existing OSes or software systems can properly manage it. We propose a new OS model called the splitkernel to manage disaggregated systems. Splitkernel disseminates traditional OS functionalities into loosely-coupled monitors, each of which runs on and manages a hardware component. Using the splitkernel model, we built LegoOS, a new OS designed for hardware resource disaggregation. LegoOS appears to users as a set of distributed servers. Internally, LegoOS cleanly separates processor, memory, and storage devices both at the hardware level and the OS level. We implemented LegoOS from scratch and evaluated it by emulating hardware components using commodity servers. Our evaluation results show that LegoOS’s performance is comparable to monolithic Linux servers, while largely improving resource packing and failure rate over monolithic clusters.