SQL> create table t (val integer) partition by range (val)
  2  (partition p1 values less than (100),
  3  partition p2 values less than (maxvalue));

Table created.

SQL> select partition_name, buffer_pool from user_tab_partitions where table_name='T';
PARTITION_NAME                 BUFFER_
------------------------------ -------
P1                             DEFAULT
P2                             DEFAULT

2 rows selected.

SQL> alter table t modify partition p1 storage (buffer_pool keep);

Table altered.

SQL> select partition_name, buffer_pool from user_tab_partitions where table_name='T';
PARTITION_NAME                 BUFFER_
------------------------------ -------
P1                             KEEP
P2                             DEFAULT

2 rows selected.