Hi
You can try this out, to transfer your OLM file to iCal format. As we all know there is no direct procedure to export OLM file to ICS file. Therefore you can first drag the Outlook calendar to the iCal icon, i.e drag the Outlook for Mac calendar to the desktop and from there drag it to iCal. From here you can easily convert your iCal file to ICS which can then be imported to gcal.
Try it, might it helps you.