Going by what I can find, while the AV output of the Neo Geo AES has the same pinout as the Genesis/Mega Drive model 1, they use slightly different DIN connectors. That makes them mechanically incompatible if outfitted with the RGB pins unless you use an adapter dongle. Check that the two pins in the DIN plug closest to the key indentation haven’t been pushed in or become bent. Many RGB cables for each console may take its sync signal from pin 7, which is one of the pins that are aligned differently (the other one is pin 6 which is used for red.)
If you don’t have an adapter, you either need one or a bespoke Neo Geo RGB SCART cable to avoid damaging the Genesis/Mega Drive one.
If, on the other hand, the cable is not outfitted with the full set of pins in the DIN plug it will fit without problem in both consoles but only carry composite video, making it incompatible with the OSSC Pro since it requires RGB (unless you outfit it with the Legacy AV In expansion to get separate composite and S-video inputs.)