Possibly modifying the columns as NULL might be a simpler approach than looking up the system names of the NOT NULL constraints and disabling them:
Code:
SQL> CREATE TABLE testit (col1 INT NOT NULL, col2 INT NOT NULL);
Table created.
SQL> set lines 50
SQL> desc testit
Name Null? Type
----------------------- -------- ----------------
COL1 NOT NULL NUMBER(38)
COL2 NOT NULL NUMBER(38)
SQL> ALTER TABLE testit
2 MODIFY (col1 NULL, col2 NULL);
Table altered.
SQL> desc testit
Name Null? Type
----------------------- -------- ----------------
COL1 NUMBER(38)
COL2 NUMBER(38)
SQL>