Hi,
I would highly recommened that you visit FreeRadioUnlocking.com
You can find their guide on generating a Honda Odyssey radio code for free here: [http://www.freeradiounlocking.com/brands/honda-radio-code/honda-odyssey-radio-code]
I can personally vouch for this website as I've used it to unlock the radio of both my Honda Civic and my wifes Honda Accord and it worked great both times.
I hope this answer helps you out!