Create table with Auto increment field in oracle

There is no such thing as "auto_increment" or "identity" columns in Oracle. However, you can model it easily with a sequence and a trigger:

Table definition:
CREATE TABLE departments (
  ID           NUMBER(10)    NOT NULL,
  DESCRIPTION  VARCHAR2(50)  NOT NULL);

ALTER TABLE departments ADD (
  CONSTRAINT dept_pk PRIMARY KEY (ID));

CREATE SEQUENCE dept_seq;

Trigger definition:

CREATE OR REPLACE TRIGGER dept_bir
BEFORE INSERT ON departments
FOR EACH ROW

BEGIN
  SELECT dept_seq.NEXTVAL
  INTO   :new.id
  FROM   dual;
END;

UPDATE: IDENTITY column is now available on Oracle 12c version, see this:

CREATE TABLE t1 (c1 NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY,
                   c2 VARCHAR2(10));