SQL> create table nn_test (col_1 date constraint nn_test_col_1_nn not null);

Table created.

SQL> 
SQL> select table_name,constraint_name,search_condition from user_constraints where table_name = 'NN
_TEST';

TABLE_NAME                     CONSTRAINT_NAME
------------------------------ ------------------------------
SEARCH_CONDITION
--------------------------------------------------------------------------------
NN_TEST                        NN_TEST_COL_1_NN
"COL_1" IS NOT NULL


SQL> 
SQL> alter table nn_test drop constraint NN_TEST_COL_1_NN;

Table altered.

SQL> 
SQL> select * from user_constraints where table_name = 'NN_TEST';

no rows selected

SQL> create table t (col number not null);

Table created.

SQL> desc t
 Name                             Null?    Type
 -------------------------------- -------- ----------------------
 COL                              NOT NULL NUMBER

SQL> alter table t modify (col null);

Table altered.

SQL> desc t
 Name                             Null?    Type
 -------------------------------- -------- ----------------------
 COL                                       NUMBER

01451, 00000, "column to be modified to NULL cannot be modified to NULL"
// *Cause: the column may already allow NULL values, the NOT NULL constraint
//         is part of a primary key or check constraint.
// *Action: if a primary key or check constraint is enforcing the NOT NULL
//          constraint, then drop that constraint.