As you may already know, Office 365 services are accessible either directly using their own URL’s (like https://outlook.office365.con or https://<you tenant>.sharepoint.com) but they are also accessible from the Office 365 portal which also allows end-users to install software, update their settings…
However, when an end-user type https://portal.office.com , he is usually redirected by default to his OWA mailbox, while admins are directed to the admin portal.
A coming update will allows you to define the landing page.
Setup your Start Page
To do this, just logon to any Office 365 services, and go to the Office 365 settings menu available from the Gear
Then click on the new option called Start Page and select your start page from the Office 365 portal to Yammer, or Outlook; off course, option to choose the Office 365 admin center is only available to administrators.
The settings will be take into account the next time user logged on Office 365.