Your best bet would be to re-import the DICOM files stored to OsiriX into Horos and let it manage building the database from the files. Horos has a much older version of the database schema used by OsiriX MD so it is not a simple migration. In Horos, import the files stored under "~/Documents/OsiriX Data/DATABASE.noindex". If there are a lot of files under that directory it may take a while.
If you need to preserve other data stored in your OsiriX MD database such as reports it will take more work.
Regards,
Todd Jensen, PhD