Convert MSO to WORD

As far as we know, such .mso files cannot be converted to anything, they actually cannot be even opened at all, so any kind of mso to word conversions seems unlikely.

MSO files can be found as e-mail attachment when the sender of the e-mail used MS Word or other programs as default text editor for MS Outlook. Usually they are only visible because the receiver of the message does not use Microsoft Outlook and contain special formatting for the actual message. This whole function is used so the recipient sees the document / message exactly the same as the sender. So your best option here is to contact the sender to send you PDF directly.

Simply put, it is impossible to convert MSO to WORD directly.
Thus, there is no so-called mso to word converter or a free online .mso to .word converter to be found.