I would not recommend Csync or composite video sync cables. Luma sync is the way to go. With composite video sync you will get diagonal lines through the video, especially with PAL signals. Luma has the same quality as Csync.
The OSSC expects a 75 Ohm terminated signal and with luma you don’t have to worry about that. With Csync you have too many things to consider: TTL sync, NESRGB/N64RGB which can also output 75 Ohm Csync, sync strippers which are incorrectly wired e.g. sync stripper in the SCART end instead of console end and not terminated to ground, PAL SNES/N64 outputting 12 volt on the Csync line damaging your equipment.