Sorry to burst into this conversation, by I’m curious why the OSSC doesn’t accept Composite signal through the Scart connector with an adapter.
Here in europe we connect our composite signals via an adapter which converts the signal into the scart input.
Here is an example
Most displays that take SCART typically accept all of RGB, S-Video, and composite (and sometimes YPbPr), whereas the OSSC only accepts RGB and YPbPr.
That adapter is a passive adapter, so it will let you put composite video on SCART, but, like Harrumph said, it does not have the capability to change composite or S-Video into a format that the OSSC will understand. The OSSC will accept composite video as a sync signal (as well as luma and clean composite sync), but it will still need those other RGB signals in order to show an image.
If you want to use consoles or old PCs whose best output capability is composite or S-Video, you will need something like a RetroTINK 2X (If you want HDMI output) or VGP’s upcoming Koryuu (if you want YPbPr output and/or to pair with the OSSC).