Quick answer
Getting an Outlook calendar into Google, or a Google calendar into Outlook, means publishing one side and subscribing to it on the other: in Outlook.com, publish the calendar to get a link, then add that link in Google Calendar with 'From URL.' Going the other direction, copy the Google calendar's private feed link from its settings and add it in Outlook with 'Subscribe from web.' Each direction flows one way and has to be set up separately, so nothing keeps both apps in sync automatically without those links in place. To see everything in one place without wiring links twice, merge both feeds into one link with Calfeed and subscribe to that once.
Outlook and Google Calendar do not talk to each other directly. Getting one calendar showing inside the other app takes a link: one side publishes it, the other side subscribes.
Outlook calendar into Google
In Outlook.com or Outlook on the web, open Calendar settings, go to Shared calendars, and under Publish a calendar, pick the calendar and how much detail to share. Publishing returns a link. In Google Calendar, select the plus beside Other calendars, choose From URL, and paste that link in. The Outlook calendar now sits alongside the Google ones, updating on its own.
Google calendar into Outlook
In Google Calendar, open the calendar's settings, go to Settings and sharing, and scroll to Integrate calendar. Copy the private address listed there. In new Outlook or Outlook on the web, select Add calendar, then Subscribe from web, and paste the link in. Classic Outlook desktop reaches the same result a different way: Home ribbon, Open Calendar, From Internet, then paste the link. That subscription keeps refreshing on its own, unlike File, Open and Export, Import and Export, which is a separate one-time file import.
Both directions, not two-way sync
Publishing links each direction gets events from both calendars showing up in both apps, but that is two one-way feeds running side by side, not a real two-way connection. A change made directly on the Outlook side, after the Google link exists, does not flow back through that same link. Each direction has to be wired and maintained on its own.
One link instead
For everything in one place without wiring links twice, paste both calendars' feed links into Calfeed's merge tool. One combined link comes back, refreshed hourly, up to 10 feeds free and 25 on Pro, and it works the same in Outlook, Google, Apple, or any app that reads a calendar link. Downstream, Apple checks about once an hour, Google every 12 to 24 hours. It is a read-only merge: edits made after subscribing do not travel back to the original Outlook or Google calendar.