This paper introduces FOS — an FPGA Operating System, which provides abstraction services for FPGA applications in an analogous way as known from software operating systems. FOS is designed as a Linux extension for ARMFPGA hybrids. The FOS management services include 1) FPGA multi-tasking and context-switching based on dynamic reconﬁguration and cooperative scheduling, 2) communication abstraction based on the ARM AMBA standard, and 3) memory isolation for privacy and security purposes. Moreover, FPGA applications deployed on FOS and the FOS kernel itself can be built and maintained independently. This is a crucial feature for higher design productivity and more ﬂexible system updates. Prototypes were implemented for latest Xilinx UltraScale+ ZCU102, UltraZed and Ultra96 platforms to demonstrate the capabilities of FOS.