I spent quite some time researching this, and as far as I can tell, the recipe for Oregano's Fat Free Sweet Serrano Salad dressing is NOT in the wild. They have been able to keep this under wraps.
I don't know if it is a trade secret, or just that nobody has asked them yet. I once asked for a recipe I didn't think someone would disclose and they just scribbled it down on the back of my bill. With that in mind, it is worth asking. Especially if you live somewhere far away from their nearest location. :-)
Here is the phone number and other contact information for Oregano's Pizza:
Oregano's Cheesequarters
7217 E. Shea Blvd.
Scottsdale, AZ 85260
(480) 829-0898
Attention: NOTHING on this site may be reproduced in any fashion whatsoever without explicit consent (in writing) of the owner of said material, unless otherwise stated on the page where the content originated. Search engines are free to index and cache our content. Users who post their account names or personal information in their questions have no expectation of privacy beyond that point for anything they disclose. Questions are otherwise considered anonymous to the general public.