Configuring preferences and options in Outlook.com
@ReynaldoB, I don't think your answer is entirely correct. I use two different instances of O365 -- one for a US Gov't agency for whom I work, and another for a university where I study (both can be accessed from www.office.com). This undesirable change has happened on the university email but is still the way it should have stayed (i.e., buttons in between the sender & subject) on the USG site. So Micro$oft still has the code active to do it the way we want ... just not everywhere we want.
And to the person who suggested the reading pane as a workaround ... BAD Idea! The reading pane is completely unacceptable for anyone who cares about the looking-over-the-shoulder element of security, and it, along with the pop-up notification containing part of the message, are disabled instantly on any new computer I approach.