There is only one file type that uses the .ORB extension:
Original eBook Reader File
An ORB file is an eBook file used by ORB (Original Book Reader), a program used to view ORB eBooks. It contains an eBook, which includes cover, title, contents, and index pages.
ORB files are compressed with Zip compression and contain .SWF files that make up the eBook. If you come across an ORB file you can open it with Original Book Reader, which is available for Windows and macOS. You can also manually view the contents of an ORB file by renaming the ".orb" extension with the ".zip" extension then extracting the contents with a Zip utility.
Updated: April 12, 2017