For PS1: All you need is an RGB SCART cable.
For PS2: All you need is an RGB SCART cable. If you wish to use 480p+ modes, you will likely want to enable automatic input switching, so the OSSC can handle the switching between 15kHz RGBS and 31kHz+ RGsB without manual intervention. Alternatively, you could continue to use your SCART cable, set output to YPbPr, and set the input on the OSSC to AV1-YPbPr, which would also work to avoid dealing with output format changes during video mode changes.
For SNES: You might be able to get away with solely an RGB SCART cable (any of sync-on-composite, sync-on-luma, or properly-attenuated CSync); however, the SNES is known to have sync jitter (a non-visible line gets cut short every other frame), which a number of modern displays will not tolerate, and which the OSSC is incapable of rectifying. I would recommend trying with just the SCART cable first. If you encounter sync problems, you will need to either add a scaler between the OSSC and your display or fit a dejitter mod to your SNES.
For N64: The OSSC only supports RGB and YPbPr, and this console only has composite video and/or S-Video output. You will need to get a decoder (like the Koryuu–if you do, get an S-Video cable for your N64 and a female stereo RCA->male 3.5mm TRS adapter); get a composite and/or S-Video to HDMI converter (like a RetroTINK 2X [Pro]) and an HDMI->YPbPr or an HDMI->VGA converter; or fit your console with an RGB or HDMI mod.