The Internet of Things (IoT) is projected to soon interconnect tens of billions of new devices, in large part also connected to the Internet. IoT devices include both high-end devices which can use traditional go-to operating systems (OS) such as Linux, and low-end devices which cannot, due to stringent resource constraints, e.g. very limited memory, computational power, and power supply. However, large-scale IoT software development, deployment, and maintenance requires an appropriate OS to build upon. In this paper, we thus analyse in detail the specific requirements that an OS should satisfy to run on low-end IoT devices, and we survey applicable operating systems, focusing on candidates that could become an equivalent of Linux for such devices i.e. a one-size-fits-most, open source OS for low-end IoT devices.