Elapsed: 00:00:00.01
SYS@mysrv>create user u1 identified by oracle;
create user u1 identified by oracle
ERROR at line 1:
ORA-01920: user name ‘U1’ conflicts with another user or role name

Elapsed: 00:00:00.01
SYS@mysrv>create user u2 identified by oracle;

User created.

Elapsed: 00:00:00.01
SYS@mysrv>select username from dba_users;

SCOTT

SYS@mysrv>conn U1/oracle
Connected.
U1@mysrv>conn u1/oracle
Connected.
U1@mysrv>conn u1/ORACLE
ERROR:
ORA-01017: invalid username/password; logon denied
@>conn u1/oracle
Connected.
U1@mysrv>create table t1 (id int);

Table created.

Elapsed: 00:00:00.05
U1@mysrv>create table T1(id int);
create table T1(id int)
ERROR at line 1:
ORA-00955: name is already used by an existing object

Elapsed: 00:00:00.00
U1@mysrv>create table t2
2 (id int);

Table created.

Elapsed: 00:00:00.01
U1@mysrv>select table_name from user_tables;

Elapsed: 00:00:00.08
U1@mysrv>

Warning: You are no longer connected to ORACLE.
@>conn u1/oracle
U1@mysrv>select * from t1;

no rows selected

Elapsed: 00:00:00.01