If you really want to use your unmodded NES with the OSSC, there’s not much you can do. You could track down an RGB decoder, like the Kramer FC-4044 (I have this; works great into the OSSC, though I get a lot of noise when hooked directly into my TV); but devices like that are hard to find these days (with HDMI and full-digital video now being the norm), they can be relatively expensive, and they’re not going to improve the picture quality of the NES’s composite video–garbage in, garbage out.
Your best bet would be to mod the console for RGB; it’ll work with the OSSC, and it’ll look great. Alternatively, you could get the High-Def NES, which would give you HDMI-out, negating the need for an OSSC. If you really don’t want to mod your console, you could always get a second-hand CRT for cheap, also negating the need for an OSSC.
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