Many are asking if they can make use of centralized auto signatures in Exchange Online in Office 365. The answer is yes - and its done using Disclaimers.
To automatically apply disclaimers to e-mail messages, you use Transport Rules. You create Transport Rules in the Exchange Control Panel - just click Manage My Organization > Mail Control > Rules and start building the desired rule.
Clicking New will present you with a dialog in which you can specify that you want the new rule to fire on all messages and you want it to append a disclaimer to evey message.
Next up is specifying the appropriate text in the disclaimer. The disclaimer text can include HTML tags and you can add user attributes to disclaimers. For example, you can add
Company to create personalized signatures. Here is an example of a (very) basic disclaimer:
When the disclaimer is added to the message, the attribute names are replaced by the corresponding values from the sender's user account.
Testing the disclamer
Compose a message (with no signature to begin with - will be inserted by the transport rule)
The recipient sees the message with the signature inserted by the Transport Rule:
Here is more on how to build your own centralized auto signatures - Link
Bonus info: Loryan Stant (Office 365 MVP) elaborates a bit on the variables and an exception you can apply - link