The Old Ones are alluded to as being Nephilim, Hyperboreans, Titans and any number of other powerful magical or supernatural creatures that have appeared in a number of books. There is no single source on information, and no canon answer as to their identity (to my knowledge).
It really depends on who you mean by the Old Ones. If you mean, as in Second Edition, the ancient race who existed before the fall of Atlantis and taught magic to humans after their betrayal by Tarhuthis, some of whom became dragons to escape the flood, well, they haven't appeared in this edition, except perhaps as the Tangential Magi who commune with Criamon magi. Or not. Or perhaps they are the guys who made the Serpent Ladon and his Garden of Secrets. Or not.
As Tim points out the legend of the old ones has not appeared in fifth edition material as of yet (and I somewhat doubt that it will).
You should know that the Ancient Magic book is explicitly about things for the seekers to go after and find. The topics of the book all concern other myths rather than the atlantis story. While Ancient Magic is not the most generally useful book ever published for the line it is a serious contender for the title of most fun book ever published for the line.
I did mean the Old Ones mentioned in my 4th Edition (but still WotC) Houses of Hermes book, the very same who were betrayed by Tarhuthis and all that.
In my saga (not the online one) the magi have found the remains of a creature and the only reference they could find of it was obscure and alluring to the Old Ones, claiming the creature, then called a Vis Serpent was often bound by the Old Ones.
I never meant for that reference to be the actual truth about the creature, but the players all liked it so much that I'm considering a change to accommodate this, but I would like to see whatever else is available about the Old Ones, other than the information I have in my 4E HoH book.
Ancient Magic does seem like an interesting supplement, and since my magi are turning into seekers themselves... I bet it was a great buy!
Page 26 of the divine book puts forth the theory that the Grigori (a tenth choir of 200 angels who stayed neutral in the conflict between Lucifer and God) were the old ones. This is presented only as a theory.