I want to stop identity of column in oracle so how to off that

There is no such a thing in Oracle. Search for the associated sequence.

Oracle supports sequences for generating primary key values; sequences are not tied to a particular column in a table. To get the same "Identity key effect" in Oracle, you can create a trigger to automatically assign sequence values to a column. This also means that there is no direct link between the sequence and the table or column. Many of commercial software application for administration and development of Oracle Database, as Toad for Oracle, already implements the procedures that combines actions and simplify this operation. At first, this sounds cumbersome. However, it is as effective and a good deal more flexible too.

More information about Sequences?
http://download.oracle.com/docs/cd/B28359_01/server.111/b28310/views002.htm[^]
http://download.oracle.com/docs/cd/E10405_01/doc/appdev.120/e10406/dialogs.htm#BACJCCFJ[^]


Cheers!