Office 365 (Outlook on the web) - Always Show "Bcc" when Composing Email
In Outlook Web App, the "Bcc" field is not shown by default when composing a message. This document explains how to enable the "Bcc" field to be displayed by default in the Outlook Web App.
After logging into the Outlook on the web, click on the Settings icon in the upper-right corner. Next, click on Mail located under "My app settings".
Within the Options menu on the left side of the screen, click on Message format under the "Layout" heading.
In the "Message Format" section, check the box next to Always show Bcc.
Click the save button at the bottom of the screen to save your settings.