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I recently was looking at GREEN LANTERN, and was surprised that O'Neil actually did a few sporadic single issues of the GREEN LANTERN series, as a fill-in writer, before O'Neil and Adams were teamed up in issues 76-89.

These are the issues :

63 by O'Neil and Sparling/Green, Sept 1968
https://readcomiconline.li/Comic/Green-Lantern-1960/Issue-63?id=28282

64 O'Neil and Sekowsky/Geilla, Oct 1968
https://readcomiconline.li/Comic/Green-Lantern-1960/Issue-64?id=28283

68 O'Neil and Kane/Geilla, April 1969
https://readcomiconline.li/Comic/Green-Lantern-1960/Issue-68?id=28287

72 O'Neil and Kane/Geilla, Oct 1969
https://readcomiconline.li/Comic/Green-Lantern-1960/Issue-72?id=28292

Interesting to see O'Neil in these earlier stories, working with Silver Age artists, in a more conventional Silver Age style, before DC basically fired these older creators all overnight, because they were gently pushing for healthcare benefits and a better living wage. And at that point DC's management had a good reserve of hungry new writers and artists like O'Neil, Wein, Wolfman, Friedrich, Chaykin, Wrightson, Kaluta, Simonson and others who were just grateful to have the work and didn't ask a lot of questions about what happened to their predecessors.

I think it was in the Summer or Fall of 1969 that DC cut loose these creators like John Broome and Gardner Fox very suddenly and replaced them with all these new writers and artists. I don't know if their replacements even knew what happened to the folks they replaced. Comics grandmasters who created virtually all of the DC characters, craftsmen who who had been the backbone of DC's line for almost 30 years, then were abruptly discarded.

But as Dennis O'Neil said in the above lengthy videotaped interview, he spent his first few years doing stories on modest titles for Marvel, Charleton and DC, where there was no pressure, where O'Neil could learn the craft of comics writing for his first 3 years or so, before he emerged as a major talent.