in Oracle Database 12c, the following are the list of system administration accounts:

and the following is the list of Operating System Accounts (OS Accounts) and its corresponding Oracle 12c System Admin Accounts:

to view all accounts in your database and their status, use the following query:

SELECT
username
,account_status
,lock_date
,created
FROM dba_users
ORDER BY username;

you can also use this query, from root container in 12c:

SELECT
c.name
,u.username
,u.account_status
,u.lock_date
,u.created
FROM cdb_users u
,v$containers c
WHERE u.con_id = c.con_id
ORDER BY c.name, u.username;