Method #1: The Bongos
Iwaizumi and Oikawa were both approached by Tokyo University, after they had lost their match to Karasuno, both were offered full scholarships to play volleyball for TU. Iwaizumi was happy that a school from one of his top choices had approached him and even happier that they were offering him a full scholarship. It didn't take much for Iwaizumi to convince Oikawa to take the offer. Now about a month into university life Iwaizumi is majoring in physical therapy, aiming to be a doctor, hoping to one day open his own clinic and help others. Oikawa is so focused on volleyball that his major is undecided, which worries Iwaizumi. However Oikawa does well in his general education classes and should be declaring his major soon.
Iwaizumi and Oikawa are in one of the library's study rooms, reviewing notes and catching up on homework, when Iwaizumi stretches and mutters something about how frustrating the math problem he had been working on for the past half hour was way too long and how he didn't get enough sleep last night. Oikawa turns to his boyfriend and jokingly asks,
"Why don't you take a nap on me for a bit? Haha just kidd-" without warning Iwaizumi lays his head on Oikawa's lap and mutters sleepily, "Thanks Oikawa, wake me up in 20 minutes, I still have stuff to do and I have class later."
Recovering from his shock that Iwaizumi actually called Oikawa without adding an insult in his name he realizes that Iwaizumi's breathing has already become slow and deep. Oikawa slowly pulls out his phone, being careful not to wake his snoozing boyfriend, and takes pictures of the vulnerable, sleeping Iwaizumi. He puts one of them as his new home screen and sets up a timer, puts on earbuds, before returning back to his own work.
Oikawa's phone goes off signaling the end of Iwaizumi's nap time. He starts poking Iwaizumi on the cheek calling his name softly, "Iwa-chan... Iwa-chan..." Iwaizumi stirs in his sleep but doesn't wake up. Oikawa tries again, this time shaking him a bit, but to no avail. Oikawa thinks hard, how can I wake him up..? Hmm maybe after this song... Oikawa, still wearing earbuds, hears that his favorite song is now playing, he turns up the volume and starts to pat along to the beat.
" -KAWA, OI SHITTYKAWA!" Oikawa's earbuds are pulled from his ears and he looks down at a very red and very irritated Iwaizumi. "TRASHYKAWA WHAT DO YOU THINK YOU'RE DOING!?" Oikawa realizes he had been using Iwaizumi's face as bongos.