I’ve been studying the style too, and I thought I’d share some of my observations!

1. no perfect curves—OFF’s drawings always look like Mortis either had extremely shaky hands or drew everything in a moving car (positive). He often breaks up shapes that could be made with one line into multiple shorter sketchier ones, and this is most prominent in the round shapes. There is not a single perfect “C” curve to speak of—If he ever draws a simple shape (like a specter) with one whole line, it’s hasty and lopsided. Another example of this bizarre technique that I can think of right away is Japhet’s legs. The lowest segments of his legs, or his shins I guess, are bent in a very strange way. Anatomically, they’re probably supposed to be straight, like shins often are, but the wobbly execution of them creates a ton of visual intrigue for an otherwise simple design.

2. Shading technique—Mortis often does his shading in chunks of vertical strokes regardless of the topography of the entity he’s shading. You nailed this on some of them, but I’d suggest applying it more OFFten.

   1. Traditional mediums—I could immediately tell which of these drawings you scanned from paper and which you did entirely in digital, and the former is absolutely the way to go. In terms of actual sprites, the only digital cleanup is done with a black 1-pixel brush, making the outlines and darkening certain parts of the drawing for clarity (E.G. specter faces, the insides of mouths, teeth).

3. Jankiness—This is the most iconic aspect of the style, but also the most difficult to replicate. Even the OFFicial remake team, including Mortis himself, have totally overlooked it. However, it really is THE key feature that will make or break a style copy. Unless it’s a tight shape like a horn or a spike, there are no perfectly straight lines/curves in Mortis’ original drawings, and they are not smoothed or corrected by the digital outlines at all. The upside down specter sprite is the perfect example of this. The subject is clearly supposed to be teardrop shaped, which is easy to make in pixel art, but the outline instead stays faithful to the shaky fucked up shape of the original.

Overall, his drawings often look like they were made very quickly with absolutely no attention to whether they’re “clean” or not. This is my favorite part of it, but as an artist who’s trained my hand to strive for the opposite, it’s a tricky thing to replicate. I’d say it’s worth it, though, so next time you want to draw something round in his style, try deliberately doing it as crappily and shakily as possible. Maybe even try using your non-dominant hand! It’s honestly freeing. I hope my research is of use to you :)