FileInfo.com - The File Extensions Resource

.MSO File Extension

File Type 1	Inline E-mail Attachment (Most common)
File Format	File
Common?	No
File Description	File sent with Microsoft Word 2000 e-mail attachments; often named "Oledata.mso;" contains information that allows the attachment to be rendered within the e-mail message. The Oledata.mso file will appear as a separate attachment on computers that do not have Microsoft Outlook 2000 or later installed.
Program(s) that open mso files	Mac: Microsoft Entourage Windows: Microsoft Outlook 2000 or later

File Type 2	Microsoft Office Macro Reference File
File Format	File
Common?	No
File Description	File created when saving a Microsoft Office document as a Web page; contains information about macros and OLE (Object Linking and Embedding) objects included in the original file; may be referenced by the created Web page as a style sheet. May be viewed with a text editor, but will be unreadable since the contents are encoded.
Program(s) that open mso files	Mac / Windows: Should not be opened manually

Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/mso