If I’m understanding your problem correctly (and you or someone else can correct me if I’m wrong), the games you’re playing purposefully don’t use the full video frame for visuals because they assume you’re playing on a CRT and that the edges will get cut off due to overscan.
Unfortunately, the OSSC is a line doubler, not a scaler. This means that, unlike the Framemeister, the OSSC cannot crop or stretch the image to arbitrary sizes and shapes. You will need to use an external video processor, or rely on your television’s zooming capabilities (if any) to stretch the image to eliminate or reduce overscan. Alternatively, there’s always the Framemeister.