This paper presents a review of standardization activities toward the interoperability between different metaverse platforms, which is a research hotspot in the metaverse. The metaverse is often considered as the evolution of the current mobile Internet. Like for the Internet, interoperability is essential to provide continuous user experiences over different metaverse service environments. However, current metaverses have limitations in providing interoperability in terms of portability of services, digital objects, such as identity and avatars, and economic systems. To address these limitations, the metaverse industry and open-source communities have recently begin to develop metaverse interworking standards. We describe activities conducted by various representative standardization bodies such as the Metaverse Standardization Forum, W3C (World Wide Web Consortium) Open Metaverse Interoperability, Open Metaverse Foundation, Open Metaverse Alliance for Web3, and Metaverse Alliance. In addition, we discuss how to counteract interoperability issues from the Korean perspective.