select tab1.usr, tab2.salary 
from tab1
LEFT JOIN tab2 ON (tab1.usr = tab2.usr and tab1.name = tab2.name);

USR   salary
-----   -------
1111  5001
2222  NULL
3333  NULL

Since usr: 2222 not exists in tab2 I am trying to get the output like below:
USR   salary
-----   -------
1111  5001
2222  NULL
3333  'NOT FOUND'

So you want to know if you joined to a row, but the column is null vs when there's no matching row in the outer joined table?

If so, check the values of the joined columns. If this is not null then display the column. Otherwise return the

You can do this in a case expression or decode:

create table tab1 
( 
usr number, 
name varchar2(10) 
);

create table tab2 
( 
usr number, 
name varchar2(10), 
salary number 
);

insert into tab1 
values (1111, 'Jerry');

insert into tab1 
values (2222, 'John');

insert into tab1 
values (3333, 'Scott');

insert into tab2 
values (1111, 'Jerry', 5001);

insert into tab2 
values (3333, 'Jerry', NULL);

commit;

select tab1.usr, tab2.usr ,
       case
         when tab2.usr is not null then 
           to_char(tab2.salary)
         else
           'NOT FOUND'
       end case_sal,
       decode( tab2.usr , null , 'NOT FOUND', to_char(tab2.salary) ) dec_sal
from   tab1
left join tab2 
on     tab1.usr = tab2.usr;

USR   USR   CASE_SAL   DEC_SAL    
1111  1111  5001       5001       
3333  3333                        
2222        NOT FOUND  NOT FOUND  

(The names for 3333 don't match, I'm guessing you these are supposed to be the same or you just wanted to join on usr...)