In Outlook:
1) Please log in to the web version of Office 365
2) Open the Outlook calendar at
https://outlook.office.com/calendar
3) Open the settings (gear wheel top right)
4) Open Calendar => Shared Calendar
5) Under "Publish a calendar" you can create an ICS link
In Calendar.online:
6) Open your administrator link
7) Open the settings (gear wheel at the top right)
8) Open "Import" in the navigation on the left
9) Click on "New Import" on the far right
10) Use the ICS link from point 5 and create a new import for Calendar.online