According to the video out specs the sync is TTL level and the RGB signals are 4 volts. The 100 Ohm resistors are not enough. They should be at least 330 Ohm for RGB. For TTL Csync 470 Ohm is usually used. Otherwise the sync voltage will be too high for SCART and may damage the OSSC.
The 470 Ohm which is already in your SCART cable is not used for Csync but for the RGB activation signal which is only for some devices like SCART TV’s. The OSSC does not use it.
You should be seeing a very bright image with your current cable.
At this page you can also see that they recommend to replace the resistors with 330 Ohm if the image is too bright which seems to confirm it. http://www.ep128.hu/Ep_Konyv/EnterFace.htm