Sync Office 365 calendar with local Outlook Calendar

I am new to Office 365 and have run into this issue.

I have been running Outlook 2010 in 'local mode' with all mail etc in a local PST file.

When I first connected Outlook 2010 to my O365 account the calendar entries sync'ed between them which was good.

I manually migrated contacts, notes and some email.

At the end of my trial period I expect to migrate over to O365 but in the interim period it is useful to keep the 2 calendars in sync.  Can this be done automatically or do I have to drag and drop from one to the other?

Regards,

Martin