The contributor that changed his name to Toru wrote "Name spelled wrong" as the reason. I checked on google translate too and apparently it's "Tōru"; episode 20 on crunchyroll is also named "Oikawa Toru is Not a Genius". But it is true that most people call him Tooru so idk really.
Regarding your question, I would think that the name is supposed to be Toru instead of Tooru because both Crunchyroll, as Wiki Contributor Healnarutoule had also said, and Wikipedia, two very trusted sites (Crunchyroll even being considered "official"), actually refer to him as Oikawa Toru. So, Oikawa Toru would probably be the best way to refer to him.
Thank you very much for asking that question as I'm sure a few other Wiki users may be confused as well, and if you have any further questions, concerns, or if you'd just like to discuss a topic, please feel free to join the Live! Chat or leave a message on either my wall or Temp. Admininstrator NoyaThePenguin's wall, and we'd be glad to assist you. As always, we thank you for all your hard work, and we hope you will stick with the Haikyuu!! franchise and continue to contribute to this Wiki in the future.
I hope this doesn't seem too weird, but I just looked through the forums again and found this, but in case anyone who stumbles across this thread is confused with our new page names, we started using the romaji-version of the names of characters, but I still think that it's pretty correct way to spell his name is with Toru instead of Tooru, although Tooru looks better to me. But since this might become a potentially debatable thing on our Wiki, do you guys think I should make an announcement for this?
I agree with the previous posts that the correct spelling is Toru. The official reading of his name (from his character profile in volume 2) is とおる. When translatated to English, the "o" receives the macron to indicate the long vowel, and thus written as Tōru. However, since "ō" is a special character and is not easy to type online, most people drop the macron.