Are you concerned that you are accidentally replying to everyone in an email when you only wanted to reply to the original sender? Change the Outlook 365 OWA defaults by following these instructions.
How to change the default Reply settings.
1. Select the Settings Menu (the gear-shaped icon in the upper right corner of Outlook 365).
2. Click on Options.
Navigate to Options and Settings.
1. Expand the Mail options.
2. Expand the Automatic processing options.
3. Click on Reply Settings.
Choose and Save your new default Reply settings.
1. Click on the radio button to the left of Reply.
2. Click on the Save button to store your changes.