Other than Genos, King, and those he's defeated(if they even knew they were fighting him), who else knows his secret? It seemed to me the Hero Assoc. still hasn't taken a real notice.
Going by the chapter 102 of the Shonen Jump Manga:
Genos, King, Silver Fang, Blizzard, and (possibly) Mumen Rider
Well, Suiryu seems to recognize Saitama's incredible power when they first meet, but only after their bout does he realize that his strength is nothing compared to baldy's. Also, down the road (Monster Association arc and onward spoilers):
After the huge battle with MA and Garou, some heroes see that Saitama is actually really OP. Some of these are Sweet Mask, Zombie Man and even Tatsumaki begins to suspect something after the battle. Also, in the latest webcomic chapters, Flashy Flash sees that Saitama is strong, but instead of respecting him he tells him that he wishes to train him in order to make him even better, not knowing that Saitama's overwhelming speed and strength really don't need any more perfecting.