Q:

not a single-group group function using select case statement

I am writing below query which divides the two select query and calculate the percentage. But i am getting an error as not a single-group group function 
select CASE WHEN COUNT(*) = 0 THEN 0 ELSE round((r.cnt / o.cnt)*100,3) END from 
    (Select count(*) as cnt from O2_CDR_HEADER WHERE STATUS NOT IN(0,1) and DATE_CREATED > (SYSDATE - 1)) r cross join
    (Select count(*) as cnt from O2_CDR_HEADER WHERE DATE_CREATED > (SYSDATE - 1)) o;

A:

You don't need to use joins. If I were you, I'd do:
select case when count(*) = 0 then 0
            else round(100 * count(case when status not in (0, 1) then 1 end) / count(*), 3)
       end non_0_or_1_status_percentage
from   o2_cdr_header
where  date_created > sysdate - 1;

Here's a simple demo:
with t as (select 1 status from dual union all
           select 2 status from dual union all
           select 3 status from dual union all
           select 2 status from dual union all
           select 4 status from dual union all
           select 5 status from dual union all
           select 6 status from dual union all
           select 7 status from dual union all
           select 1 status from dual union all
           select 0 status from dual union all
           select 1 status from dual)
select case when count(*) = 0 then 0
            else round(100 * count(case when status not in (0, 1) then 1 end) / count(*), 3)
       end col1
from   t
where 1=0;

      COL1
----------
         0

And just in case you aren't sure that doing the filtering of the count in the case statement returns the same as when you filter in the where clause, here's a demo that proves it:
with t as (select 1 status from dual union all
           select 2 status from dual union all
           select 3 status from dual union all
           select 2 status from dual union all
           select 4 status from dual union all
           select 5 status from dual union all
           select 6 status from dual union all
           select 7 status from dual union all
           select 1 status from dual union all
           select 0 status from dual union all
           select 1 status from dual)
select 'using case statement' how_count_filtered,
       count(case when status not in (0, 1) then 1 end) cnt
from   t
union all
select 'using where clause' how_count_filtered,
       count(*) cnt
from   t
where  status not in (0, 1);

HOW_COUNT_FILTERED          CNT
-------------------- ----------
using case statement          7
using where clause            7

