It has been a month or two since I did this, but I don’t recall doing anything special on the OSSC.
I have this setup:
A CGA2RGB rev B01 and a OSSC is 1.6 with firmware 0.78.
C-128 RGBI -> CGA2RGB -> VGA OSSC -> HDMI out to a DELL 2407WFP
I have the jumper set to pins 2-3 (closest to the board edge).
That will set the CGA2RGB to output separate sync RGBHV.
I direct plugged the 128 to the CGA2RGB.
I ran a VGA cable from that to the OSSC.
The monitor is connected to the OSSC with a HDMI cable with a DVI-D adapter on the monitor.
From what I recall, the OSSC autosensed the RGBHV signal and worked out of the box.
I don’t think I set Line2x or 3x.
Is there a 5V power supply properly connected to the CGA2RGB?
If none of that helps, let me know and I’ll set it up again and list out any other settings.