@unime I don't know, but it's likely Murata. It could be ONE though. It'd be funny. What I do know however is that while Murata draws the art, the story(including the new elements of the story that weren't in the webcomic) and the fight choreography, as well as the character designs(before they are given more details by Murata) are all done by ONE.
As such, since the story-board is done by ONE, it's actually possible for ONE to have personally drawn the OPM webcomic-like shots of Tatsumaki and Saitama.