Regarding SCART on the OSSC: You should be setting the input to AV1-RGBS; AV2 is the RCA/component input, so you should expect a black screen/no input if you’re trying to get an image using the SCART jack and have the input set to something else.
Regarding the N64: The N64 only shipped with composite video and S-Video outputs, while the OSSC only takes RGB and YPbPr. You would need to mod your N64 to get RGB output if you wish to use it with the OSSC, or to get HDMI output if you’d like to skip the OSSC and connect it directly to your flat panel TV (UltraHDMI).
Regarding the SCART cables: If you still don’t get an image setting the input to AV1-RGBS, double-check your cables to ensure that they’re actually wired for RGB. I doubt the N64 one is, because, as I said, that shipped without RGB support, so that is likely a composite-video-only cable and thus incompatible with the OSSC (the green sync light will come on, and you’ll get audio, but no video).
I imagine the Wii should be fine, but that might very well also be a composite-only SCART cable. If it does work, though, you will be limited to 576i50 and PAL60. Personally, I recommend foregoing a SCART cable and getting a YPbPr component cable for the Wii, as that will let you play compatible games in 480p60, which will produce a better, more fluid image.