He was definitely an artist it took me a while to appreciate. A very distinctive style, combining the madcap chaos of British humour strips with heavy metal/ sci-fi frenzy.
While I was buying 2000AD, his work in Nemesis and ABC Warriors was past, but I'd see the odd (usually harrowing) pin up of some twisted creature (Torquemada usually).
I didn't take to Metalzoic, mainly for the art. But 2000AD released Best of Monthlies at some point. So I got time to see more of his work in one sitting, and get used to the style. not only did I start of get it, bu I realised that a lot of the concepts that underpinned the later strips came from his collaborations.
Marshall Law was a bit more restrained, as chaotic as it was, by his early standards. Definitely easier to get into and enjoy the satire.
As I got a bit older, I'd get more and more from his early work, from musical to topical references. All setting up nicely for LoEG. There are a number of comic professionals who just let their creativity run wild on 2000AD. O'Neill was one of the very early ones, and I can see his approach influencing many of the others to let loose a little more on their own work. That led to the comic being such a massive, long lasting hit. I was reading a Judge Dredd comic just yesterday, partly plotted/written by the now departed Alan Grant. On the back cover is a typically demonic looking advert from O'Neill for Nemesis.
My thoughts are with his family, friends and all of the fans who have loved his work for decades.