A virtually unknown O'Neil story, but a great one, is a
PHANTOM STRANGER story, issue 8, August 1970, with art by Jim Aparo. I love it on a lot of levels, a race of creatures dormant for thousands of years, re-awakened by human civilization, with social conscience and environmental themes. And a race of creatures that have an almost supernatural aspect to them.
Like many of O'Neil's stories, a one-off story that could easily have been expanded into an entire series.
Also in this issue, the first house ad for Jack Kirby's move to DC from Marvel, and reference to the multiple new series he would be producing. Not specifically naming Kirby's Fourth World or NEW GODS or JIMMY OLSEN, but certainly whetting your appetite for whatever was coming.
O'Neil's single PHANTOM STRANGER issue was followed by one of the earliest series runs by Gerry Conway, who was still in high school writing these issues, that follow well the same compelling themes as O'Neil's story.
Roughly the same period Conway wrote a story in SAVAGE TALES 1, the first appearance of Man-Thing. Conway was only 16 at that point. I'm not a huge Conway fan, but that manifests a remarkable level of ambition and talent.
Likewise O'Neil. O'Neil modestly brought a level of elegance and sophistication, mature themes and social conscience to the titles he was assigned to take over. Whether DETECTIVE COMICS, WONDER WOMAN, SUPERMAN, BEWARE THE CREEPER, SHOWCASE, JLA, WEIRD WORLDS, THE SHADOW, GREEN LANTERN or whatever.