I tried unplugging every device in the power strip but the OSSC and PSOne (to clarify since it does have an external power adapter). No luck. If you’re right and it is a power issue, could it be the OSSC’s power supply? It does seem pretty cheap, and I ordered it from a Spanish retailer instead of videogameperfection proper, but everything that came with it seems pretty legit and I have had no similar problems with other systems. I’m using an official PSOne external power brick and cables.
As a side note, I also can’t use Line 5X mode at all with the PS1 in 263p, but I can with the SNES… Is that normal? Aren’t they both 240p? Or does the SNES just not overscan as much, if that’s the correct term? Thanks for your answer by the way, I appreciate the timely response, and 9 Volt!