create table try_departement (dep_id number(2), dep_name varchar2(30));

INSERT ALL
INTO try_departement VALUES  (1, 'SALES')
INTO try_departement VALUES  (2, 'CEO')
INTO try_departement VALUES  (3, 'MARKETING')
INTO try_departement VALUES  (4, 'IT')
INTO try_departement VALUES  (5, 'SECURITY')
INTO try_departement VALUES  (6, 'HR')
SELECT * FROM DUAL;

SQL> select * from try_departement
  2  order by decode(dep_name, 'CEO', 1, 'IT', 2, 'SALES', 3, 4), dep_name
  3  /
    DEP_ID DEP_NAME
---------- ------------------------------
         2 CEO
         4 IT
         1 SALES
         6 HR
         3 MARKETING
         5 SECURITY