I dont think this is the manga where that would happen. Obinata got just as jealous immediatly after and she just heard them talking. They're the same in that regard and immediatly went to comfort each other.
Only one with the misunderstanding is their friends, even then its nothing to worry about
I sorta see where the other person's coming from, if only because of the one-shot.
But there's also the need for
some amount of conflict/tension within the narrative, and one of the tried'n'true tropes of that for school-age romcom manga is "the potential triangle/love rival".
Making Osaka (seemingly) completely oblivious to the entire situation (as far as the romance hypothetical goes) fully couches all of the conflict in the MC's own mind, meaning that the actual potential for problems is fully imaginary at this point.
And the FMC is
fully head over heels for the MC, AND she's clearly also the type to get jealous, given her reaction to the fourth friend giving her boyfriend a pet name at the Karaoke bar. There's
no way she'll look askance at someone else, even if there's some potential plot point for the MC misunderstanding something (like the tomato thing in the cafeteria in an earlier chapter, or the incidental proximal closeness between the FMC and Osaka in this chapter).
The one friend having the misunderstanding that the FMC has a crush on Osaka is probably the biggest wrinkle, because if she tries to
really act on it and play matchmaker, that
could cause actual problems for our Main Pair, insofar as they're trying to keep things a secret because the MC believes that their physical disparity would cause problems for the FMC if their peers found out. But if anything, I hope that the friend tries something soon because I think the four of them knowing the Main Pair are dating would be a good thing.
(Especially because I'm shipping the Oil & Water side duo and then they can go double dating.)