Difficult to say. As much as I struggled with some bosses in Souls and other modern games, if I could relive my experiences with NES games from my youth, I guess any Souls boss would feel like a cakewalk compared to bosses from Castlevania, Megaman, TMNT, Zelda 2 etc. Bosses in classic space shooters are also much harder on average than bosses from action games, except you have lives and continues so they feel easier. The fourth, fifth and final boss from Ikaruga are way worse than O&S if you’re going for 1cc.
In recent times, I may have ragequit at Genichiro and some other similar stuff, but I seriously think some fights in Smash Ultimate’s story mode are several times harder that FS bosses.