OPS$ accounts

We have users utilizing a VB front end to log into the database. When they are logged in, SYS.AUD$ shows their username as OPS$/. However, when viewing DBA_USERS there are no usernames listed as OPS$.

I realize OPS$ is used for OS authentication, but I always thought that a user had to be created in the database as OPS$?

Anyone know why the username is being listed in SYS.AUD$ in this manner?

Thanks.

Execute the following statement:
select name, value from v$parameter where name = 'os_authent_prefix';

This is the value which is prefixed with the os username for the oracle authentication. For example if value = "OP$" and the os username is "ABC", then the oracle authenticates it as "OP$ABC"