My main retro source is ps2…
The vast majority of PS2 games are 480i, and OSSC is not really suitable for interlaced sources when you can’t do passthrough, as you’ve seen; however, you do have a few options:
1. Try to get a monitor that will do 480i (passthrough) and/or 1440i (line3x) over HDMI (what you’re doing now);
2. Try to force games to 480p using GSM, with or without Open PS2 Loader (avoid interlacing altogether);
3. Get a secondary video processor that will deinterlace (VP30, VP50, DSC 301 HD, etc.);
4. Get a Framemeister (Doesn’t excel at 480p, but does great with 480i).