grant create session, create table, create any directory 
  to testu identified by testu;
grant read, write on directory tmp to testu;

conn testu/testu

create table t (
  x varchar2(10) 
) organization external (
  default directory tmp
  location ('test.txt')
);

select * from t;

X         
----------
test

drop table t purge;
create or replace directory testdir as '/tmp';
create table t (
  x varchar2(10) 
) organization external (
  default directory testdir
  location ('test.txt')
);

select * from t;

X         
----------
test