Many-core architectures trade single-thread performance for a larger number of cores. Scalable throughput can be attained only by a high degree of parallelism and minimized synchronization. Whilst this is achievable for many applications, the operating system still introduces bottlenecks through non-local sharing, synchronization, and message passing. A particular challenge for highly dynamic applications, for example invasive HPC applications and elastic compute clouds, is the management of short-living application threads and processes. This paper discusses OS architecture choices based on microkernel, multikernel and distributed systems designs and our development experience in the context of the MyThOS project. Initial experiments show a much faster thread creation and activation compared to monolithic systems like Linux while providing a more flexible protection and threading model that is better suited for dynamic scenarios. However, despite significant progress in the overall domain of operating systems, the design space for scalable many-core operating systems is yet to be fully explored.