I possibly have an fault with the HDMI to VGA adapter (sold with by VGP for RetroTink -> OSSC connection) or have made connection / OSSC configuration foobar. I have fiddled about trying to get a sync on AV3 and hitherto have not had any luck. I note that the output direct from Tink’s HDMI to a display works fine (testing simply for retrotink color bar output).
To figure out whether it’s the VGA converter (adapters are passive) or the OSSC, you should test the HDMI to VGA converter on a different VGA-compliant display or video processor, and you should test AV3 on the OSSC with a different VGA source. And definitely make sure that you have the input on the OSSC set to AV3-RGBHV.
Quick update I managed to find a VGA display and can rule out the OSSC AV3 input since the VGA display also has no signal. Since the output from HDMI is working and the input VGA is ruled out it does sounds like the VGA converter is faulty?