Ōyama (大山, Ōyama) is an ordinary young male who has no special talents. He is the third character to join the SSS. His roommate in the afterlife world is Fujimaki (formerly Hinata) and he was the first character to be suspected of being an NPC.
Ōyama is a noticeably average looking boy with medium length dark reddish hair, large, innocent-looking green eyes, and fair skin. He wears the afterlife school uniform in the manga. In the anime, he wears the standard SSS uniform. He tucks his shirt in and buttons his it up to the top, leaving his blazer undone, and wears a tie. He seems to be shorter and less muscular than a lot of the other boys. He says he's not very athletic, preferring to watch sports rather than play.
He does not excel at anything but can do what any normal person can do. In other words, he is a jack-of-all-trades. He is very sensitive and easily gets embarrassed in fake confessions. He has been in the afterlife world for longer than Hinata and Yuri, but most likely not as long as Shiina.
- Socially, he does not stand out much to the point that Hinata mistook him for an NPC (even being given the nickname "Villager A" during their first meeting); it was only during Hinata's third conversation with him that he clarifies his status as a dead person. Though he admits that he was once fearful of Yuri (because of what she did to Hinata), he helped Hinata in his relationship with Yuri. When Hinata became his roommate, his first impression of him was that he was incredibly sweet and nice, but that there was absolutely nothing interesting about him, as if he had been made from a template. Ooyama does his best to do his assigned tasks and does what he is told (unless he thinks it is morally wrong or scary), seemingly happy to be needed, but he is very forgetful and messes up very often, making him somewhat unreliable. He is timid and very easily frightened, and the others think of him as a wimp or a scardey-cat. He is affectionate to his closest friends, and in the manga is often seen clinging to Hinata in someway, usually when he's scared, though sometimes he clings so much that Hinata starts to get annoyed. He is generally reserved and somewhat shy, usually staying quite when the group is talking to a new person, but he's still friendly and warm. Ōyama is very innocent, childlike, enthusiastic, optimistic and cheerful when compared to the rest of the group. He seems to see the best in others and is very forgiving, as seen when Yusa killed him cruelly and he forgave her easily because he thought for someone to do something like that they must have some reason. He is overly emotional and easily hurt, and cries very easily. Because of this sensitivity, he is prone to being emotionally scarred and traumatized, which is a running gag in the series. When Takamatsu was turned into an NPC in front of him, he felt terrible and blamed himself, even though there was nothing he could do. The whole situation upset him a lot, and he seemed to find it hard to accept that it had happened and they couldn't do anything. He seems to have a strange imagination, and has very weird and sometimes unrealistic ideas and jokes( in an audio special Yuri even yelled at him for "going off into his own little dimension again"). He says he believes in true love and takes his crushes very seriously. He is generally gentle, encouraging and incredibly caring. He seems to like helping and caring for people who have suffered badly in the past (like Fujimaki). He pays attention and has good intuition when it comes to how others must be feeling, and likes it when others are happy. He hates any kind of conflict and violence. At the beginning of the manga, he was all alone, with no friends, and was very touched that Hinata actually wanted to be his friend. It seems the main reason he joined the SSS was because he didn't want to be alone. In the manga, he 'casts' pretend healing and protection spells on Hinata. Whenever the group needs to test or demonstrate something dangerous, they use Ōyama ( for example,when Yui wanted to try her new wrestling move on Hinata he said that he'd ask Ōyama if she could try it on him).
How he died and ended up in the Afterlife remains unknown, but in the manga, he is frequently shown to have sad and dramatic expressions whenever the others are discussing the past. His mood always drops immensely after talking and refused to tell about it, which implies that it is a harsh one—although there is currently no proof of this.Ooyama said that the first present ever was Hinata-kuns Natto. Due to him joining the SSS to avoid loneliness implies that he was lonely and had no friend whilst alive. When he says that Hinata was the first to give him a present could mean that his parents were to poor to give him Christmas presents, or simply didn't love him enough. His anxiousness when made to confess his love to Kanade proves that he hasn't had past relationships and is inexperienced in this field. There could also be a possibility that he played many MMO's while alive as he pretended to cast healing spells and potions on the others. His parents must have been very strict and disciplined him often or he was a natural goody-two-shoes as he was overwhelmed by eating in class. His hostility toward violence and conflict suggests that he was either bullied or abused when living.
In the anime, in Episode 2 it could be assumed that everyone already shares their past stories with each other since Iwasawa mentioned that she already know Yuri's (albeit, in no full length). Whether Ōyama already shared his story too or not, is unknown.
In Episode 5, Yuri made him confess to Angel (later known as Kanade Tachibana) he said to Hinata, "You just got away with physical damage, but I'm going to take critical mental damage." He also said, " This is my first time asking out a girl!" Hinata and Ōyama keep on fighting until Yuri shouts, "Enough! Knock it off all of you!"
In Episode 6, Ōyama did an amusing scene where he was told to do something not-normal and eat snacks in class to see what former substitute school president Naoi would do. So he specifically said, "Oh man, My heart is pounding! I'm eating snacks in the middle of class! I ate it! I totally ate it! Boldly eating snacks in the middle of class! I'm so reckless!" That could show that he was possibly picked on and forced to follow the rules in his past.
In Episode 12, he helps the others from the shadow using the sniper.
In Episode 13, he and most of the SSS are revealed to have passed on.
Though normally tasked with sniping duties where his M24 SWS variant of the Remington 700 rifle comes into use, in times of emergencies, he can also pull out his SIG-Sauer P226R pistol. It is not known if he has improved his close combat capabilities since braining himself with a flail type weapon soon after he first met Yuri and Hinata. In the anime, he is kind of a sharp shooter.
- Ōyama has the same voice actor as Yukine Miyazawa's little brother from Clannad. Although this character only appears during Yukine's Arc.
- During Episode 8 Dancer in the Dark, Ōyama was hypnotised by Naoi to act as a sacrifice, under the role of a clown to make "the lonely girl in the corner" laugh.
- His name literally translates into large mountain, but he isn't tall.
- In Angel Beats! Track Zero Chapter 1, Hinata once called him 'Yampaii'.
- In Angel Beats! Drama CD 3, his personality has changed to the exact opposite of the one he shows in the Anime.
- In the OVA2, he was also out of character as he was happy that he managed to successfully kill Takamatsu and that he recovered extremely fast after seeing Matsushita's bloody limbs and vomiting.
- Hinata thinks that he is very cute.
- Yuri seems to find his jokes hilarious in an audio special.
- His hobbies are reading and listening to J-pop, which Hinata states is as boring as you can get.
- He has a habbit of saying the same sentence twice in the same conversation.
- In the visual novel, he thinks Otonashi is handsome and wants to nickname him ' Ikeman', which could be translated as " handsome" or " handsome guy".
- In the manga and drama CDs, he has the catchphrase "Nante ______!", which could be translated as " Such ________!" or " What ________!". His most common phrases could translate as " Such violence!" and " What survival skills!"
- He loves nattou, which a lot of people (including Yuri) hate.
- He's very popular with the lunch ladies.
- In a life before his previous one, he was a huge playboy, with the complete opposite personality. He says he " Messed around with some older women" and talks about how he misses his favourite hotels and bars, though he isn't even old enough to drink anyway. This is a running joke in the drama CDs, in which he occasionally swaps to his opposite personality
- He begged to be friends with Fujimaki after he heard his past and felt such strong sympathy from him he was brought to tears.
- His birthday seems to be in either October or September
- In Angel Beats! OVA 2 (Hell's kitchen) TK says to Ooyama "Hey, you!" in English, then in Japanese says, "Right now, she's waiting at the airport lobby," which could be a possible hint of his past. (Either his or TK's)