The MSO file extension primarily refers to files created with Microsoft Office 2000, and more specifically to files created with Word 2000. When an HTML message is sent from Microsoft Outlook along with an Office 2000 attachment, the message is sent in the form of an Outlook HTML message, with an MSO file attached.
The MSO file allows other users to render the HTML message correctly on their computer within the email message itself, even if they are not using Microsoft Outlook software. This process is also referred to as "round tripping." Computers that do not have Outlook 2000 installed will display MSO files as additional attachments, and these files are usually attached under the file name "Oledata.mso." The MSO file is not editable and does not contain any readable information; it is just there to enable the Microsoft Word attachment to be viewed by other users.
Deleting the MSO file can cause the attached file to be rendered unreadable, so it is important to not remove these files. The MSO file extension can also refer to charts created with the Microsoft Organization Chart tool. Organization charts can be created using numerous Microsoft Office programs, including Word, Excel and PowerPoint, among others. Charts created using these programs are also saved with the MSO file extension.