In Oracle 12.2, We can split partitions or subpartitions online without impacting the DML statements.

SQL> select partition_name,read_only,high_value from dba_tab_partitions where table_name='ORDER_TAB';

PARTITION_NAME                   READ HIGH_VALUE
-------------------------------- ---- --------------------------------------------------------------------------------
CREATED_2105_P10                 NO   TO_DATE(' 2015-11-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIA
CREATED_2105_P11                 NO   TO_DATE(' 2015-12-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIA
CREATED_2105_P12                 NO   TO_DATE(' 2016-01-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIA
CREATED_2105_P8                  NO   TO_DATE(' 2015-09-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIA
CREATED_2105_P9                  NO   TO_DATE(' 2015-10-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIA
CREATED_MX                       NO   MAXVALUE

6 rows selected.

SQL> alter table order_tab split partition CREATED_MX  into
(partition CREATED_2106_P2 VALUES LESS THAN (TO_DATE('01/03/2016', 'DD/MM/YYYY')),PARTITION CREATED_MX) ONLINE;  2

Table altered.

SQL>  select partition_name,read_only,high_value from dba_tab_partitions where table_name='ORDER_TAB';

PARTITION_NAME                   READ HIGH_VALUE
-------------------------------- ---- --------------------------------------------------------------------------------
CREATED_2105_P10                 NO   TO_DATE(' 2015-11-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIA
CREATED_2105_P11                 NO   TO_DATE(' 2015-12-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIA
CREATED_2105_P12                 NO   TO_DATE(' 2016-01-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIA
CREATED_2105_P8                  NO   TO_DATE(' 2015-09-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIA
CREATED_2105_P9                  NO   TO_DATE(' 2015-10-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIA
CREATED_2106_P2                  NO   TO_DATE(' 2016-03-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIA
CREATED_MX                       NO   MAXVALUE

8 rows selected.