Have you checked your TV’s settings? Your post makes it sound like the OSSC is plugged into a different screen than if you just plug the Dreamcast in with VGA. Am I correct?
To clarify, I’m trying to make it work on my TV (TCL 40FS3800), which only accepts HDMI, so I can only do Dreamcast -> VGA Box -> OSSC -> TV (HDMI). I tried testing Dreamcast -> VGA Box -> PC Monitor (VGA) and the image was fine.
The OSSC’s black-and-white test screen, on my TV via HDMI, does not have any artifacting or vertical lines. It looks perfectly fine.
I adjusted my TV’s settings, and by turning “Sharpness” all the way to 0, the image did get a bit better, but the vertical lines and visual artifacting are still there, just more subtle: https://i.lensdump.com/i/8svb03.jpg https://i.lensdump.com/i/8svwB0.jpg
EDIT: So apparently according to this thread, this is normal, and what I’m seeing is just the dithering effect brought out by the high quality HDMI conversion of the OSSC, that you wouldn’t normally see? https://i.lensdump.com/i/8sLMUe.jpg is this normal?