SQL> create table tt1( c1 varchar2(10), c2 varchar2(10) );

Table created.

SQL> create table tt2( c1 varchar2(10) );

Table created.

SQL> insert into tt1( c1, c2 ) values ( 'test1', 'test2' );

1 row created.

SQL> insert into tt1( c1, c2 ) values ( 'test3', 'test4' );

1 row created.

SQL> insert into tt2( c1) values ( 'test1' );

1 row created.

SQL> select * from tt1, tt2 where tt1.c1=tt2.c1(+) or tt1.c2=tt2.c1(+);
select * from tt1, tt2 where tt1.c1=tt2.c1(+) or tt1.c2=tt2.c1(+)
                                                       *
ERROR at line 1:
ORA-01719: outer join operator (+) not allowed in operand of OR or IN

SQL> select * from tt1 left join tt2 on ( tt1.c1=tt2.c1 or tt1.c2=tt2.c1 );

C1         C2         C1
---------- ---------- ----------
test1      test2      test1
test3      test4

2 rows selected.

 select max(decode(chk,'1',column_1)) column_1,max(decode(chk,'2',column_1)) column_2
   from
   (          
   select 1 column_1 ,'1' chk from dual
   union
   select 2 column_2,'2' chk from dual          
   )