In OWA, as happens with Outlook, you can add any shared MailBox where you have permissions:
Navigate to your OWA in Office 365 and select the Inbox folder in your Mailbox so a contextual menu is shown. From this menu you can add a shared Mailbox by clicking on the Add shared folder… option.
In the modal dialog that opens, search for the shared mailbox you want to add and then press the Add button:
Then, you will see the Shared Mailbox added to your OWA folder structure. Of course, you will be able to access to any e-mail sent to the Shared Mailbox and also send e-mails from it if you are granted to.