When sending HTML messages with an attached Microsoft Office 2000 document, the message is sent as an Outlook HTML message, not an Office Envelope message. The Oledata.mso file contains necessary information to properly render the attachment in the message in its originating program. This is known as "round tripping." There is no useful information for the user in this file and it is not editable. Do not delete this file; doing so could render the attached file unuseable.
If an HTML message is opened on a computer that does not have Outlook 2000 installed, you will see the Oledata.mso file as an additional attachment.