To add a signature in Office 365 Outlook, you can follow these steps:

1. Open Outlook 365.
2. Click on the "Settings" gear icon in the top right corner of the screen.
3. Click on "View all Outlook settings" at the bottom of the Settings panel.
4. Select "Compose and reply" from the left-hand menu.
5. Scroll down to the "Email signature" section.
6. Type your signature in the text box provided.
7. Use the formatting options to change its appearance.
8. Click "Save" when you're done.

You can also create a new signature by opening a new email message, clicking on "Signature" > "Signatures" > "New".