Who's This? The oldest character in Who's Who.
How you could have heard of him: The New52 did make him show up in Justice League Dark, Constantine and others, as the keeper of the House of Secrets, but was killed off within a year. He was just glimpsed in the Lazarus Planet event.
Example story: More Fun Comics #29 (February 1938) "The Spectral Killer" by Jerry Siegel and Joe Shuster
It's a short one all right, but then, they all were. The nature of New Fun, reprinting a lot of newspaper comic strips along with some new material, meant Dr. Occult usually had a single page to himself, and his stories wrapped up in a couple pages. Late in his run, he gets a whopping FOUR pages, and it's still very cursory, because Shuster opts for big panels instead of condensed, claustrophobic layouts. Superheroes displaced him - More Fun would still make them occult; it was the home of Dr. Fate and the Spectre - and because his stories ended so early, he was largely forgotten even when occult detectives made a comeback. Comics starring Dr. Thirteen and the Phantom Stranger could have been his big revival. It wasn't until comics historian Roy Thomas dug him up 50 years after his debut that readers started getting interested. I still am. He's got a great name, for one thing. And all the stuff with Rose Psychic sharing his body now seems resonant in this post-gender world. Who knows, he might get another shot.
Who's Next? A radioactive skeleton.