While being logged in as SYS, I created a new user. I ran
select username from dba_users order by username;
and I was able to see the user that i just created;
Then, I logged as SYSTEM, I ran the same query, but the user was missing this time. This leads me to the question: don't all users have the same instance of the table DBA_USERS? I thought that this was a system related table, and all users get the same instance (as long as they have the right privileges to access this table).