Code:
CREATE TABLE EMPLOYEE
( Fname VARCHAR2(25) NOT NULL,
Lname VARCHAR2(25) NOT NULL,
Ssn CHAR(9) CONSTRAINT SSN_PK PRIMARY KEY,
DOB DATE,
Address VARCHAR2(30),
Sex CHAR,
Salary DECIMAL(10,2),
Super_ssn CHAR(9),
DEPT_ID  NUMBER 
 );

Table created.

ALTER TABLE EMPLOYEE
ADD CONSTRAINT DEPT_ID_FK FOREIGN KEY (DEPT_ID) REFERENCES DEPARTMENT (DEPT_ID) 
ADD CONSTRAINT
...