Expanding on what Harrumph said, SCART is designed to accommodate all of composite, S-Video, and RGB, but the receiving device may not support all of them. In the case of the OSSC, the OSSC can only support RGB (including just about every sync type out there) and YPbPr (what most people refer to as ‘component’, often not carried by SCART).
In order to be used with the OSSC, Composite and S-Video need to be decoded to RGB or YPbPr by using a device specifically for this purpose, such as the RetroTINK 2X, which accepts SD video over composite, S-Video, or YPbPr component, and decodes to RGB over HDMI (which you can then convert to VGA for use on the OSSC).
Composite and S-Video to SCART adapters, like the one you have, are passive adapters, intended solely for allowing you to connect composite and S-Video sources a SCART input on a TV or other video device when you don’t have a native SCART cable. They simply put the video signals on the correct wires for SCART; the receiving device still needs to know what to do with those signals, and the OSSC does not.