I was having a look at a Zapier and tried one out. I choose Outlook Calendar to Google Calendar because I am not an Outlook fan but I have to use it for work. I use IMAP to get my work emails and as a result, forgo the calendar sync. It works pretty well and I got it up and running surprisingly quickly. Big tick here.
Interested to know how it is done?
The secret is in the Outlook Exchanges API to grab the calendar events and the in the Google calendar API to push the events to. A quick Google search for Microsoft Exchange API will take you to the right place or you can use this link: http://msdn.microsoft.com/en-us/library/office/jj984343(v=office.15).aspx. Click if you want to go directly to the calendar API docs.
Note that you can’t actually just use the API. There is some set up do such as registering your app and reading the API docs to fully understand how it works. This is what Zapier obscures hence the advantage of using Zapier. There is a price to be paid though.
A quick search of “Google Calendar API” will also lead you to a similar destination where you can POST the calendar data to Google.
However you do this, you then just set up a scheduler to poll a set interval to grab the data.