Assuming you added your Outlook.com account to System Preferences>Internet Accounts and enabled your calendar to sync with the Apple Calendar app, whatever events you add in the Calendar app will be saved to your Outlook.com calendar (assuming you selected the event to be saved under your Outlook.com calendar).
If, for whatever reason, you had to setup your Mac as a new device, all your Outlook.com calendar events will show up if you re-enable the calendar sync in the above mentioned preference pane.