First, since your TV (somehow) doesn’t have aspect ratio controls, if you use the OSSC, you will probably need to go line3x on 240p/288p content, because line2x (480p/576p) outputs a 4:3 image, which will likely cause the image to be horizontally stretched to fill the screen (assuming your TV doesn’t respect aspect ratios).
Secondly, the OSSC only has a single SCART input, so you will need a SCART switcher; preferably an automatic one, like the gscartsw or Hydra, since you’re aiming for simplicity.
Lastly, since you didn’t mention whether your NES or SNES have been modded, your NES (And your SNES, if it’s one of those mini/Jr models that only outputs composite and RF) will need either an RGB/YPbPr mod or some other way to convert the composite output from the NES to RGB/YPbPr (VGP’s Koryuu, composite/S-Video to component converter, RetroTINK 2X plus an HDMI to component converter); and, if your TV ends up being particular with sync signals (a lot are), you may need to get a dejitter mod for the SNES (this also applies to the NES, but the NESRGB 2.x revisions have dejitter baked in).