The classic "red box" that, for me, was my introduction to D&D, featured Otus' work on the cover. It's an iconic image that will forever scream "Dungeons and Dragons."
Otus is again doing work for RPGs, thanks to patronage from the "old school renaissance." There's a great tribute site here with lots of images, although it's sort of wonky to navigate and you've gotta dig for good stuff. But if you've got a few spare minutes, you could spend them in worse ways than digging back through some of these classic images.
I think my favorite EO illustration is the one from the back cover of the classic module The Keep in the Borderlands (it came with the red box basic set). While it doesn't feature his trademark stylized figures, I think it captures the mood of an impending adventure better than anything else I've seen. I love the sky and the way it reflects on the walls of the keep - and there are our brave adventurers, seeking out their fortunes or their dooms. This illustration is pregnant with possibilities, and I think it ends this entry nicely.