In the period from 1978-1982, DC used to run "DC Profiles" of its writers and artists on their editorial pages.
Here is Dennis O'Neil's, from WARLORD 8, Sept 1977 (and other DC titles the same month) :

Quote
DC Profiles # 17
DENNY O'NEIL

If you're the type who reads the small print on the publisher's page, it is safe to assume you know who Denny O'Neil is. It's remotely possible you don't read GREEN LANTERN GREEN ARROW, or you never fell across one of Denny's classic BATMAN tales, or never experienced his scripting on SUPERMAN, JLA, WONDER WOMAN or the others, but you still know who he is.
But you don't know the whole story. Denny is a prolific writer, starting out his professional life as a reporter and feature writer in Missouri, and moving on to such magazines as OUI, CORONET, GENTLEMAN'S QUARTERLY (a fashion magazine), PUBLISHER'S WEEKLY and a host of others.
Denny has also written reams of short stories for the likes of AMAZING, FANTASTIC, THE MAGAZINE OF FANTASY AND SCIENCE FICTION, and ELLERY QUEEN'S MYSTERY MAGAZINE. He has written several novels, a book on the presidents (CONTEST FOR POWER) , and was editor for a news magazine.
How does a guy with these credits find himself an award-winning comic book writer?
It all started out in Missouri 12 years ago, when Denny interviewed Roy Thomas, who at the time was a big-name comics fan. When Roy ultimately wound up at Marvel [1966], he sent Denny a writers test, which Mr. O'Neil passed. After 6 months as an editorial assistant [and writer at Marvel], Denny started frelancing at Charleton for then-editor Dick Giordano. Giordano offered his writers unheard-of editorial freedom. It was during this period that Denny wrote his personal favorite, an S-F thriller called "Children of Doom" [ CHARLETON PREMIERE 2, Nov 1967].

When Dick Giordano moved to DC, he brought Denny with him, Abandoning his "Sergius O'Shaugnessy" pseudonym (now it can be told!) O'Neil started working on such classics as BEWARE THE CREEPER and BAT-LASH. He's been here ever since.
Denny would like to try his hand at filmwriting, or maybe even do a mainstream novel. If he handles it with the same sort of flair he's shown at DC the past nine years, comics fans won't be the only ones who know who Denny O'Neil is.

"Sergius O'Shaugnessy" was the pseudonym O'Neil used at Charleton, and also his first year or so at DC. When I spoke to O'Neil about it at a convention, he said he selected that name because it was a character in Norrman Mailer's book Armies of the Night, a book that was a favorite of his, which he also referenced in his GREEN LANTERN stories. A pseudonym probably used because he was under contract at Marvel, and couldn't have his name appearing in Charleton and DC titles at the same time. A pseudonym he likely discontinued once his contract at Marvel ran out. BEWARE THE CREEPER ran 6 issues, and that pseudonym was used through the first 3 of those issues. He began transitioning to Denny O'Neil with issue 3.

Another good early O'Neil story is "Abra-Cadoom" , the second story in THE SPECTRE 9, April 1969, that was the second pro story done by Berni Wrightson. Another title edited by Dick Giordano.

Here's a chronological list of O'Neil's other early credits.
http://mikesamazingworld.com/mikes/features/creator.php?creatorid=128

O'Neil was a real trooper, paying his early dues at Marvel, writing a ton of MILLIE THE MODEL stories for years (issues 138-153, June 1966-Sept 1967) , and a lot of western stories. The first thing I read of his early Marvel work were his Doctor Strange stories in STRANGE TALES 145-149 (June-Oct 1966), and 167-168 (April-May 1968), roughly coinciding with Steranko's "Nick Fury, Agent of SHIELD" stories running in that title.

It was over the next 3 or 4 years that O'Neil prolifically went on to :

BEWARE THE CREEPER 1-6, after the debut story in SHOWCASE 73 by Ditko and Segall, 1968-1969.
THE SPECTRE issue 9, "Abra-cadoom", art by Wrightson, 1969.
SHOWCASE 82-84 (Nightmaster), 82 with Granenetti/Giordano art. 83-84 with Wrightson art, 1969.
WONDER WOMAN (issues 178-181, 199-202), 1968-1969, 1972.
SUPERMAN 233-242, 244, 247, 1971-1972.
JLA 66-83 and 115, 1968-1970, 1974.

and ultimately
DETECTIVE COMICS (issues 395-411, 414, 418, 419, 422, 425, 431, 451, 457, 481) , 1970-1980.
BATMAN 224-266, 268, 1970-1975
and GREEN LANTERN 76-89. 1969-1972, and issues 90-110 with Grell, and 111-129 with Staton, 1976-1980.