Hi TT4,
That's a tricky one! AccountRight files (such as .MYOX) are not well-suited to be stored in OneDrive or other cloud-synced folders. Every time a change is made in AccountRight, OneDrive attempts to sync the updated file. Because AccountRight files are large and frequently updated, this can result in OneDrive creating multiple versions or duplicates, sometimes appending the PC name to avoid conflicts. More information can be found here: OneDrive is full - AccountRight files
I would also suggest to submit a support a support request on this help page MYOB Help me OneDrive, then scroll down to Submit a Request.
Regards,
Earl