You can access your Outlook Web Application (OWA) settings by clicking the gear icon near your profile picture. Settings specific to your email are listed, and you can also access settings for all of Office 365 from here.
Automatic Replies can be accessed through the gear. From here you can customize start/end times, custom messages, and the people that are sent the automatic replies.
These basic display settings allow you to customize the way that messages, conversations, and the reading pane appear within OWA.