Hi, Mike,
I'm no "expert" like some of the guys on here, but I do know a little about the "oath of allegiance".
Missouri was under martial law at the time. Any Southern sympathizers could take the oath and avoid SOME of the persecutions they were being subjected to. Also, any person who worked as a member of a crew, or owned a Mississippi or Missouri River steamboat was required to take the oath in order to continue operating during the war. So, one possibility would be that he was employed on a steamboat since the Quartermaster utilized the steamboats to transfer troops and supplies. Can you share anything more of your ancestor's background?
Best regards,
Maggie
"steamboat granny"