os_username vs username in Oracle audits
The os_username is the name of the user that runs the application. The username is the name of the user defined in the database.

Suppose that UserA is logged on his desktop as UserA and runs an application to modify the data. When he is asked to connect he enters the credentials of UserB. In the audit you will find UserA as os_username and UserB as username.

This explains while you find oracle as os_username since this is the user that is used to run Oracle itself.