alter session set nls_date_format = 'hh24-mi';

with pretest as (
    select 8 start_time, 11 end_time, 'P1' name, 1 id, null owner_id from dual
      union all
    select 10 start_time, 10.5 end_time, 'P2' name, 2 id, null owner_id from dual
      union all
    select 11 start_time, 15 end_time, 'P3' name, 3 id, null owner_id from dual
      union all
    select 11 start_time, 12 end_time, 'D1' name, 4 id, 3 owner_id from dual
      union all
    select 13.5 start_time, 14 end_time, 'D2' name, 5 id, 3 owner_id from dual
      union all
    select 12 start_time, 13 end_time, 'P4' name, 6 id, null owner_id from dual
  ), test as (
    select trunc(sysdate) + start_time / 24 start_date 
         , trunc(sysdate) + end_time / 24 end_date 
         , name, id, owner_id
      from pretest
  )
----/\--- sample data ----/\---
select start_date, end_date, name, id, owner_id
     , first_value( decode( owner_id, null, start_date ) ) over( partition by nvl(owner_id, id) order by start_date ) owner_start_date
  from test
 order by owner_start_date, owner_id nulls first, start_date, end_date

     , max( decode( owner_id, null, start_date ) ) over( partition by nvl(owner_id, id) ) owner_start_date