By watching this OVA, I'm going to assume that you have watched the original series and that you know the ending. If not, then this review will be a big spoiler for you.

(There will be some SPOILERS in my review)

[Story]: (5/10) (SPOILERS)
A special episode of "Waiting in the Summer" included in the Blu-Ray Box Set. The events of the OVA takes place at their final year of High School.

I wish the OVA took place after the events of the main series because this special episode didn't really add anything. It's just a look back to the events of their summer 2 years ago with some "new" scenes stamped into it. Notice the quotation mark? That's right, they're not really new. They're just fanservice scenes that we have already seen so many times in the original series, scenes showing how they've moved on and some re-hashed jokes. For example, Remon making them drink something that looks like a lemon and later, they would get drunk. Those kind of jokes. While I do agree that the jokes are repetitive, they're still somewhat funny. To me, at least.

At the last 2 minutes of the episode, we're shown that Ichika has been granted access to return back to Earth after M.I.B reversed-engineered a ship that allowed them "interstellar travelling", coinciding with the main series' ending. How disappointing.
[Art/Animation]: (9/10)
The animation looks exactly the same as the main series. Same visuals, same animation, same character designs. In other words, it didn't really change and it didn't need to change.
[Sound]: (10/10)
The soundtrack....Simply outstanding. The OP theme song is "Kisetsu no Shutter" by Ray and the ED theme song is "point at infinity" by Nagi Yanagi. The OP theme song is just beautiful. At the time I'm writing this review, I can't stop listening to the OP song and I need someone to stop me. The singer's voice and the song is just awesome. The ED theme song also sounds great.

All of the character's VA reprises their role so no VA replacement for the current main characters. Oh, thank god.
[Characters]: (6/10)
Since the OVA takes place right before Ichika has been granted access to return to "Earth", only the 5 main characters makes an appearance and Ichika appearing at the ending. There were a couple of side characters who makes an appearance like Kaito's older sister and Tetsurou's older sister.

Since you already know the characters, I can't really say anything that much else about the characters. Except we're shown that Tetsurou and Mio are now dating, Kanna moved on over her love for Kaito, Remon is still with the M.I.B and the reasons are still unknown as to how she managed to join the agency in the first place and Kaito living his life without Ichika. They're all back to normal and happily ever after. Kaito's older sister and Tetsurou's older sister is still the same old, same old.
[Enjoyment]: (6/10)
I did enjoy the special episode.....a little bit. Mostly because of the scene where they "accidentally" get drunk. Ahh, it never gets old.
[OVERALL]: (5.4/10)
"Waiting in the Summer: Special" is a decent OVA but you don't have to watch it unless you really want to know more about the character's life after the main series. Nothing important really happened. If you're a big fan of "Waiting in the Summer" then I guess you can watch this short episode too. Otherwise, just use your time on something else.

+Great Animation and Visuals
+Outstanding OP and ED theme songs

-Mediocre Special Episode
-OVA should have taken place after the main series' ending
-OVA didn't add anything new
-Repetitive jokes
-Repetitive fanservice scenes