It is very straightforward for end user to configure their Safe Senders and Blocked Senders list in OWA both for Exchange 2013 and in Exchange Online.
On the main page of OWA, select the gear icon at the top right of the page and select Options:
From the left hand pane of the Options panel, select block or allow:
In this page, users can add the desired sender(s) or domain(s) to the Safe Senders or Blocked Senders list by typing the e-mail address or domain name and selecting the add icon. Once all of the entries are added, scroll down to the bottom of the page and select save.