||Outlook In-Line Office Attachment
||Referenced by an application; not meant to be opened manually
||★ ★ ★ ★ ☆ Common
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.
Information from the PC File Extensions Reference