You’re using that adapter to connect a composite or S-video signal to the OSSC, is that right? I’m pretty sure that’s just a passive adapter, so it’s probably just passing those to the composite, chroma, and luma pins on the SCART connector. Remember, SCART can carry a bunch of different video formats, but the video digitizer IC Marqs used for the OSSC is only compatible with RGB and YPbPr, not composite or Y/C.
Unfortunately, a passive adapter won’t work: you’d need an active adapter (e.g., http://www.ebay.com/itm/Composite-Video-to-RGsB-RGB-Component-Video-Converter-/120332220654?rmvSB=true) to convert those into something the OSSC can use, and even then you’re stuck with the lousy quality of composite. It looks like there are some sort of inexpensive passive cables on eBay that are supposed to convert Y/C S-Video to YPbPr component (which, in theory, the OSSC could use on AV2), but I haven’t the faintest idea how that’s supposed to work without an active transcoder.
Honestly, what you really want to do is just set things up to pull RGB directly from the consoles. The SNES, at least, should be good to go for that already.