Is it possible to automatically back up my mac to Microsoft OneDrive?
IF the onedrive is mounted then you can use sync and cron with a script
(Please note on kindle fire and format may be off so this will be simple):

cd $HOME
rsync -ar . /Volumes/&lt;mount point of one drive&gt;


This has more details:

Once you have saved script, and gave it execute permission, use crontab -e to add your frequency.  

Please see this for crontab example (I can not give proper screen display with kindle)  

If the drive is not visible with df, then I am not sure. 

Following up on this answer, you could use this utility to mount your OneDrive folder and to the backup using the info above: