Sorry if this has been asked before, Fair Class (O)

What does fair class O mean. I googled and most of the sites don’t list O as a class. I’m flying from home at HNL to visit SLC. I know the hard product is Delta One, but will I get lounge access and what’s the difference between O and D classes. When booking it said first class at some point and Delta One at another. Just wondering what to expect. Thank you!!