SQL> create table t ( c1 int, c2 int );

Table created.

SQL> insert into t values (1,1);

1 row created.

SQL> insert into t values (1,null);

1 row created.

SQL> insert into t values (null,1);

1 row created.

SQL> insert into t values (null,null);

1 row created.

SQL>
SQL> alter table t
  2  add constraint CHK check ( not ( c1 is null and c2 is null ) )
  3  enable novalidate;

Table altered.

SQL>
SQL> delete from t where c1 is null and c2 is null;

1 row deleted.

SQL>
SQL> alter table t
  2  modify constraint CHK
  3  enable validate;

Table altered.

SQL>
SQL> insert into t values (null,null);
insert into t values (null,null)
*
ERROR at line 1:
ORA-02290: check constraint (MCDONAC.CHK) violated