We often get the question: "For Office 365 organizations in a hybrid Exchange deployment (some users on-premises, some users in Exchange Online), what Free/Busy viewing and Calendar sharing options are available?"
In the below table you'll see whats avaliable in an organisation with a mix of Exchange Server On-Prem (2010 SP1 or 2007 SP2 or 2003 SP2) and Exchange Online (Office 365)
Note 3: Exchange 2010 RTM coexistence with Exchange Online (Office 365) is not supported.
Note 4: All scenarios require Rich Coexistence for free/busy information viewing to be possible. Simple Coexistence provides no free/busy viewing.
See also: "Exchange Calendaring Sharing Matrix" - link