Can you share the url you are using to access the Messages? If you are using something like: https://outlook.office365.com/ews/odata/Me/Inbox/Messages you should be seeing the Body property in the response. You can also specifically request selective properties using $select. For example: https://outlook.office365.com/ews/odata/Me/Inbox/Messages?$select=Subject,Body&$top=1
$top can be used to restrict the number of items returned.
If $select is specified, it will override the Accept="odata.metadata=full" setting and will return just the fields specified in the $select query.