Oracle 23c

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select
to_timestamp('153312', 'hh24:mi:ss') as time_column
from dual

TIME_COLUMN

01-NOV-23 15.33.12.000000000

alter session set nls_timestamp_format = 'dd.mm.yyyy hh24:mi:ss,ff6'
select
to_timestamp('153312', 'hh24:mi:ss') as time_column
from dual

TIME_COLUMN

01.11.2023 15:33:12,000000

select
to_timestamp('153312', 'hh24miss') as time_column
from dual

TIME_COLUMN

01.11.2023 15:33:12,000000

select
to_char(to_timestamp('153312', 'hh24miss'), 'hh24:mi:ss') as time_column
from dual

TIME_COLUMN

15:33:12

select
substr('153312', 1, 2) || ':' || substr('153312', 3, 2) || ':' || substr('153312', 5, 2) as time_column
from dual

TIME_COLUMN

15:33:12

select
to_dsinterval('PT' || substr('153312', 1, 2) || 'H' || substr('153312', 3, 2) || 'M' || substr('153312', 5, 2) || 'S') as time_column
from dual

TIME_COLUMN

+000000000 15:33:12.000000000

alter session set nls_date_format = 'YYYY-MM-DD HH24:MI:SS'
select
date '2021-02-03' + to_dsinterval('PT' || substr('153312', 1, 2) || 'H' || substr('153312', 3, 2) || 'M' || substr('153312', 5, 2) || 'S') as date_time
from dual

DATE_TIME

2021-02-03 15:33:12