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That depends. If they each are doing separate activities (hosting different mailboxes, or one is a CAS, etc) then they can be different OSes. But if you are running a DAG, that is a failover cluster underneath and thus needs to be OS consistent, like any failover cluster.
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That depends. If they each are doing separate activities (hosting different mailboxes, or one is a CAS, etc) then they can be different OSes. But if you are running a DAG, that is a failover cluster underneath and thus needs to be OS consistent, like any failover cluster.

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My environment would be as such: Server1 and Server 2 are CAS\Mailbox and Windows 2008. They will not host a DAG. Server3, 4 and 5 would be the DAG on Windows 2012.
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My environment would be as such: Server1 and Server 2 are CAS\Mailbox and Windows 2008. They will not host a DAG. Server3, 4 and 5 would be the DAG on Windows 2012.

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Do all of my Exchange 2013 servers need to be running the same operating system?<br />
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I have two CAS\Mailbox Exchange 2013 servers running OS Windows 2008 R2. Can I add three more Exchange 2013 servers running Windows server 2012 or do they also have to be on Windows serve 2008?
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Do all of my Exchange 2013 servers need to be running the same operating system?

I have two CAS\Mailbox Exchange 2013 servers running OS Windows 2008 R2. Can I add three more Exchange 2013 servers running Windows server 2012 or do they also have to be on Windows serve 2008?

Can I have a mixed OS exchange 2013 environment?

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Windows Server 2012 is the server version of Windows 8 and the successor to Windows Server 2008 R2. Windows Server 2012 is the first version of Windows Server to have no support for Itanium-based computers since Windows NT 4.0. Windows Server 2012, now in its second release (Windows Server 2012 Release 2) includes Foundation, Essentials, Standard and Datacenter, and does not support IA-32 or IA-64 processors.

Windows Server 2012

Windows Server 2008 and Windows Server 2008 R2, based on the Microsoft Vista codebase, is the last 32-bit server operating system released by Microsoft. It has a number of versions, including including Foundation, Standard, Enterprise, Datacenter, Web, HPC Server, Itanium and Storage; new features included server core installation and Hyper-V.