I’m not sure SCART can output 480p with the Dreamcast. Technically, Scart can go as high as 1080p (according to wikipedia) but I think YOU HAVE to use a VGA cable to output 480p when it comes to Dreamcast. Unless you have RGBHV through the SCART but I doubt it.
Hmmmmm this comes from Beharbros : ” Euro Scart connector for connecting to your TV or upscaler. Supports 480i & 480p.”
So maybe OSSC can’t do RGBVH through SCART. On my remote control, here are the sync options :
So, I think OSSC doesn’t accept RGBVH through SCART. Wait for a more knowledgeable person to confirm, but it seems to me that the issue comes from here.