This is how to show a colleague's calendar next to your own. Without special sharing, only free and busy times are visible.
Outlook (classic)
Go to Calendar.
Choose Home › Add Calendar › From Address Book ….
Search for the person, select them and confirm with OK.
New Outlook and Outlook on the web
Go to Calendar.
On the left, choose Add calendar › Add from directory.
Search for the person and click Add.
Tip: For more details, the person must share their calendar: right-click the calendar › Sharing and permissions (classic: Share › Calendar Permissions).