orclz>
orclz> create view scott.emp_v as select * from scott.emp
  2  where
  3  sys_context('userenv','current_user')=ename or
  4  sys_context('userenv','current_user')='SYSTEM';

View created.

orclz>
orclz> conn scott/tiger
Connected.
orclz> select * from emp_v;

     EMPNO ENAME      JOB              MGR HIREDATE                   SAL       COMM     DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
      7788 SCOTT      ANALYST         7566 1987-04-19:00:00:00       3000                    20

orclz> conn system/oracle
Connected.
orclz> select * from scott.emp_v;

     EMPNO ENAME      JOB              MGR HIREDATE                   SAL       COMM     DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
      7369 SMITH      CLERK           7902 1980-12-17:00:00:00        800                    20
      7499 ALLEN      SALESMAN        7698 1981-02-20:00:00:00       1600        300         30
      7521 WARD       SALESMAN        7698 1981-02-22:00:00:00       1250        500         30
      7566 JONES      MANAGER         7839 1981-04-02:00:00:00       2975                    20
      7654 MARTIN     SALESMAN        7698 1981-09-28:00:00:00       1250       1400         30
      7698 BLAKE      MANAGER         7839 1981-05-01:00:00:00       2850                    30
      7782 CLARK      MANAGER         7839 1981-06-09:00:00:00       2450                    10
      7788 SCOTT      ANALYST         7566 1987-04-19:00:00:00       3000                    20
      7839 KING       PRESIDENT            1981-11-17:00:00:00       5000                    10
      7844 TURNER     SALESMAN        7698 1981-09-08:00:00:00       1500          0         30
      7876 ADAMS      CLERK           7788 1987-05-23:00:00:00       1100                    20
      7900 JAMES      CLERK           7698 1981-12-03:00:00:00        950                    30
      7902 FORD       ANALYST         7566 1981-12-03:00:00:00       3000                    20
      7934 MILLER     CLERK           7782 1982-01-23:00:00:00       1300                    10

14 rows selected.

orclz>

SQL> create or replace procedure p
  2  is
  3  begin
  4    dbms_output.put_line('USER='||user);
  5    dbms_output.put_line('SESSION_USER='||sys_context('USERENV','SESSION_USER'));
  6    dbms_output.put_line('CURRENT_USER='||sys_context('USERENV','CURRENT_USER'));
  7  end;
  8  /

Procedure created.

SQL> grant execute on p to test;

Grant succeeded.

SQL> conn test/test
Connected.
TEST> set serveroutput on
TEST> exec michel.p
USER=TEST
SESSION_USER=TEST
CURRENT_USER=MICHEL

PL/SQL procedure successfully completed.

create view scott.emp_v as select * from scott.emp
where sys_context('userenv','SESSION_USER')=ename 
or sys_context('userenv','SESSION_USER')='SYSTEM';

or 

create view scott.emp_v as select * from scott.emp
where user =ename 
or user = 'SYSTEM';

create view master_table_v as
select * 
from master_table
where company = sys_context('USERENV', 'CLIENT_INFO');