Does an IOT guarantee order in a select? We need to add a priority column to a table that gets hit about 250 times a second, approx 170 selects, 125 inserts and 60 updates. The column will be a simple number(1).

The priority does not matter for the inserts or updates i.e. not part of the primary key, which I'll enforce separately.

We basically don't want to have to do an order by over a range scan 170 times a second, as the number executed will drop massively.

Does an index organised table guarantee that priority = 1 will always come before priority = 9 when running the following query:

select *
  from my_table
 where rownum &lt; 2


For slightly more context a typical query would be:

select *
  from my_table
 where mod(to_number(to_char(tstamp,'ss')),1) = 0
   and done is null
   and country = 'gbr'
   and rownum &lt; 2


The pk constraint, for the IOT, would become priority, rest of the pk with a separate constraint on the pk solely for structure. done is null in approximately 99% of the table so this isn't very selective anyway.

Main index used is, I think, country, done, to_number(to_char(tstamp,'ss'), we tested about 20 combinations and this came up top by a long way.

I am completely unwilling to add any time at all to these queries, 0.01s a day added by select is 41 minutes a day. We'd much rather settle for "good enough" than perfection.
No. 

The only thing that guarantees result set order is an ORDER BY clause in your query. 

This is a popular question about SQL so it's worth repeating what I've written in response to similar questions about SQL Server and MySQL:


  In the SQL world, order is not an inherent property of a set of data.
  Thus, you get no guarantees from your RDBMS that your data will come
  back in a certain order -- or even in a consistent order -- unless you
  query your data with an ORDER BY clause.


In Oracle, an Index-Organized Table (IOT) will minimize the amount of work the database has to do to get you the data sorted in the index's sort order.  Though you may find that Oracle tends to return rows selected from an IOT in that same order, you are only guaranteed that order when you query the IOT with an ORDER BY clause.