A new optional prerequisite for Exchange 2016 is the installation of Office Online Server (OOS).
OOS help users to see attachment in the email to open directly in the web browser, it also eliminate user having an office product install on the machine previously.
Office Online Server is available for download as part of a volume licensing agreement
You can configure an Office Online Server endpoint in two places in Exchange 2016:
A) Organization level
B) Mailbox server level
Organization There are a couple of reasons why you might configure the Office Online Server endpoint at the organization level:
Single-server or single-location deployment: You can configure the endpoint at the organization level if all of your Exchange 2016 Mailbox servers are in the same location and you don't plan on having geographically distributed Office Online Server servers.
Fallback for large deployments: You can configure endpoint at the organization level as a fallback if the endpoint configured on a Mailbox server isn't available. If an Office Web Apps server isn't available, the client will try to connect to the endpoint configured at the organization level.
If you have Exchange 2013 servers in your organization, don't configure an endpoint at the organization level. Doing so will direct Exchange 2013 servers to use the Office Online Server. This isn't supported.
- you want to distribute client requests between two or more Office Online Server
- you want to geographically distribute Office Online Server servers.
- you have Exchange 2013 in your organization, configure the endpoints at the Exchange 2016 Mailbox server level.
When you configure an endpoint at the server level, mailboxes located on that server will send requests to the configured Office Online Server.
Ref : "https://technet.microsoft.com/en-us/library/mt170644(v=exchg.160).aspx"