This is a common misunderstanding. The "SOP" is the name of a book, the American Standard Of Perfection, copyrighted and published and only sold by the American Poultry Association, the APA. You can purchase the book on the APA website.
What you appear to be asking is what is the standard for the OEGB. That standard, as well as all other breed standards, are what are bound into the Standard of Perfection along with lots of great poultry information that is worth the price of the book. The American Bantam Association also publishes their Standard book but only covers the bantams. The APA covers both Large Fowl and bantams.
You may, perhaps, find snippets of the standard floating around the web, but quoting large potion of the written standard violates copyright laws and is to be avoided. The clubs for the breed would be immensely valuable and helpful to you and I'm sure with three clicks and Google you could find those clubs and their helpful information.