I have this chinese scart/hdmi to hdmi converter too.
Here is some findings:
The chinese box accepts only limited range (16-235) from HDMI, so when TX mode on the OSSC set to HDMI the greyscale will be clipped since the OSSC output full range (0-255) always.
So set the OSSC to TX DVI mode, but the gain and offset still need to be adjusted to not clip and properly scale colors.
Set them like so: TX DVI, Offset 138, Gain 26, Pre-ADC Gain 5.
Now the greyscale and colors will be correct (as it can do) through the chinese unit.
I should point that the chinese unit adds LAG of about 4 frames (it does chroma encoding, rescaling etc..), and has screen tearing and skipped frames.
So I’d use it only as a last resort for capturing, but would never actually play through it.