Files with the extension MSO contain an inline e-mail attachment. This means that they contain a Microsoft Word 2000 document that was sent as an Outlook HTML message with Word. The file, which is mostly Oledata.Mso contains all the necessary data to correctly display the document within Outlook.
If the message is displayed on a system that does not have Outlook 2000 installed, the oledata.Mso file appears as an attachment.
The file can not be used or edited on its own and should not be deleted as this can make the corresponding message unreadable.