Automated driving has the same limitations as human drivers because it functions as a replacement for humans and operates based on local information using onboard sensors and computers. Cooperative automated vehicles (CAVs) are expected to achieve both safety and efficiency, which could not be achieved by imitating human driving, by sharing sensor information from roadside equipment and other vehicles. Since such sensor information is enormous, it is desirable to utilize mm W, which are capable of high-capacity transmission. However, wireless communication systems for CAV have the challenge of radio quality degradation due to vehicle movement. Our research project aims to realize stable millimeter-wave transmission by incorporating open radio access network (O- RAN) and vehicle-to-everything (V2X) functions. This paper presents the overall proposed concept and an example of validation; we show the results of evaluating our previously proposed fast beam following scheme in a handover environment with multiple roadside units.