Exchange Online has been extended to take advantage of One Drive for Business as a location to access or store email attachments. Users of Outlook Web Access may notice that they have the ability to now attach files from One Drive for Business (ODFB) when composing a new message.
Read more at source: http://blogs.technet.com/b/timmcmic/archive/2015/06/14/office-365-exchange-online-disable-one-drive-for-business-integration.aspx