I have so many feelings about Oikawa Tooru needing glasses (mostly fueled by own recent glasses shopping trip).
He wasn’t wearing glasses in his childhood flashbacks, so I imagine him needing them later, around late middle school or early high school. Imagine the day he goes to the doctor and they officially tell him he needs glasses - he doesn’t even need a strong prescription, but those late nights reviewing game footage in his dark room have finally caught up to him (just like Iwa-chan told him would happen). He calls Iwaizumi as soon as he leaves the doctor’s office and starts wailing into the phone, “But Iwa-chan, my IMAGE.” Iwaizumi hangs up on him (and a few more times after that when Oikawa calls back to keep complaining) but eventually agrees to go shopping for glasses with him.
Iwaizumi regrets that decision when he’s at the eyewear store the next day for three hours with Oikawa because he insists on trying on every single pair of glasses in the store. The shop girls can’t stop fawning over Oikawa and basically play dress up with him until he comes back to a pair of black half-rim glasses (probably a super nice brand like Ray-Bans too bc Oikawa Tooru is definitely a fashionista).
Iwaizumi was probably making jokes about the different glasses before he got annoyed at how long this all was taking. And Oikawa hated it at first but then started leaning in to it and picking the silliest pairs to try on just to see what Iwaizumi would say (“Hey Iwa-chan, what about these?” “You just aged 80 years.” “I think they look quite good!”) (I’m also not going to say that the half-rim glasses were probably the only pair that Iwaizumi didn’t say anything negative about and that’s probably why Oikawa got them)
And don’t even talk to me about what a baby Oikawa would be when he tried to put in his contacts for the first time (he probably got so frustrated that he gave up, went to practice without them, sucked, and then Iwaizumi wheedled it out of him was was off, then stood over him as he tried putting in his contacts again until he got it)(“You can face down Ushiwaka but you can’t put in CONTACTS?”)