Select/Group by/Order by question

I'm trying to create a summary log by hour. Here's the query (somewhat
simplified):

select to_char(mtrantime,'mm-dd hh AM') as datetime,
count(*) as tot from memtran
group by datetime
order by datetime;

The problem is this produces the data in the following order:

datetime | tot
-------------+-----
04-08 01 PM | 14
04-08 02 PM | 15
04-08 03 PM | 23
04-08 07 AM | 8
04-08 08 AM | 54
04-08 09 AM | 30
04-08 10 AM | 11
04-08 11 AM | 10
04-08 11 PM | 7
04-08 12 PM | 10

What I'd really like is to get it in chronological order by hour:

04-08 07 AM | 8
04-08 08 AM | 54
04-08 09 AM | 30
04-08 10 AM | 11
04-08 11 AM | 10
04-08 12 PM | 10
04-08 01 PM | 14
04-08 02 PM | 15
04-08 03 PM | 23
04-08 11 PM | 7

I would prefer not to show the time of day in 24 hour format, but
there doesn't appear to be a way to order by something that
isn't in the select and group by clause and I don't want to display
the hour twice.

Putting the AM/PM before the HH (which looks a bit clumsy) almost works,
except that 12PM gets sorted to the bottom after 11PM.

Is there an easy way around this?
--
Mike Nolan