If you use a non RGB-modded NES:
An unmodded NES only outputs composite video. Even if you use a SCART cable: it then simply outputs composite video via SCART. This won’t work with the OSSC, since the OSSC only accepts an RGB or component signal as video input. The SCART pins used for composite video are used for the sync signal of the RGB video input too… that’s why the OSSC may recognize the sync signal but it can’t process the composite video signal, so it won’t output anything:
It is possible to mod the NES to output RGB video:
That way you can use the OSSC directly with your NES.
Or you buy a RetroTINK 2X:
… and use it instead (or in combination with) the OSSC. The RetroTINK 2X supports composite input – so it’ll work with an unmodified NES. But it will be the lower-quality-solution, since the output of an RGB modded NES is vastly better than the composite output of an unmodded NES).