Abstract: A file management system for interfacing a host computer application or operating system to a write once read mostly optical data storage system. The file management system causes a predetermined number of sectors on the optical disk to be reserved for a directory region, and causes files of information data to be written to consecutive sectors of an information region adjacent the directory region. Directory data associated with the files of information data is then written to consecutive sectors in the directory region. The information region is thereby allowed to expand to any necessary size until directory data associated therewith fills the adjacent directory region. These procedures are then repeated, resulting in a plurality of interleaved directory and information regions on the optical disk. The file management system causes directory data to be organized in a tree structure. Each file of information data is associated with a subdirectory.