Good news about the dx - this panel bollocks is getting ridiculous though - hope you're still pressing ahead with the Tribunal.
I would still go for the 2nd opinion from CAMHS though. Having read your letter about DS, I would have said that there's more going on than just ODD.
Plus, ODD is a bit of a descriptive label rather than one which actually leads anywhere. The ASD/ ADHD label, in your case, will help with the school unit, and I would have said that you have a good reason to ask CAMHS to check for the this again with the second opinion assessment. ODD does not count as ASD (though can be a co-morbid dx), and, as I said, doesn't really account for all of the issues that your DS has.
A child I know is dx with this and there are no services to address ODD, not like with ASD and ADHD, and nor does it officially count as a disability. So I think it's used to describe 'defiant child' rather than to signpost any actual help, iykwim.
It's moving though, at last! You should claim an hourly rate from the LEA for all the time that you have to put in...hey, that's going to go in a letter to my MP: force LEAs to pay parents for their time...they might start writing correct statements to save paying us for our time then!