I’ve got my PS2 set to YPbPr, not RGB. Sorry – that was a typo in my post.
I found out what likely has something to do with the issue. When using setting my OSSC for 2x-BOB or PASSTHRU for 480i input (3x-LACED isn’t compatible for me), the problem goes away. Which is weird because I swear I’ve checked this already. Up until now my OSSC has been set to 4x-BOB for 480i input. While my TV doesn’t officially recognize the input as 960p (when pressing info it doesn’t mention the resolution), it displays it fine, so I figured it was working how it was supposed to.
So it seems the problem is my TV is mishandling the 960p input in some way, which isn’t surprising. While it’s a little disappointing to have to use 480p on a PS2 through the OSSC, I’m fine with it, since 240p sources are really what I got it for (which display GREAT in 5x mode btw). It would be great if I could get the colors on 4x-BOB working correctly, but it’s probably not possible.