The first time that you sign in to OWA (Outlook Web App), you will be asked to set your current time zone.
If you don't set the time zone, the time zone is set to (UTC) Monrovia, Reykjavik. There's no method for admins to set up the time zone setting in advance in Outlook Web App for all users.
The following screen shot shows the Outlook Web App sign-in screen in Office 365.
To change the time zone to the correct setting in OWA (Outlook Web App), follow these steps: