They had limited interactions during the anime and OVA. However, neither of them could get along with one another. In both moments, Flash would instigate a verbal conflict whenever Metal Bat is talking. This usually ends with Flash mocking Metal Bat and Metal Bat's temper escalating the conflict between them.
While there was never a true interaction between the two, Darkshine and Flash are known to be a dangerous combo in battle. The two stood together against Garou alongside Puri Puri Prisoner.
Flash mistook Saitama as a monster and the two had a brief scuffle. After clearing the misunderstanding, the two became acquaintances with one another, with Flash acknowledging Saitama's superior abilities.
After being defeated by Garou, Flash was one of the few who regained consciousness and witnessed Saitama defeat Garou. In addition to acknowledging Saitama's strength, Flash decided to teach Saitama some fighting techniques.
Flash and Tatsumaki have an unpleasant relationship with one another, just like with other S-class heroes. However, he appears to not be afraid of her and is willing to scold her back whenever she insults him.