OVA 1 is the first OVA episode of the Karakai Jōzu no Takagi-san anime series.
Waterslide (ウォータースライダー )
Nishikata and Takagi join Nakai and Mano at the water park. Upon meeting up, Nakai and Nishikata decide to go on the big water slide, much to Mano's disappointment. Now alone with Takagi, Mano explains that she is too nervous to be alone with Nakai, and that the two had agreed to ride the big slide together, following a rumor that couples going down the slide together will fall in love. Wanting to support them, Takagi decides to leave Mano alone so that she can spend time with Nakai. Shortly after, Takagi finds Nishikata wandering aimlessly, and the two go and buy juice together. Upon seeing Nakai and Mano ride down the big water slide together, Takagi challenges Nishikata to catch her on the way down; although he accepts and nearly reaches her, she tells him the slide's legend, causing him to intentionally slow himself down before crash landing at the exit. By the end of the day, Nakai and Mano ride off on a bike together, while Nishikata and Takagi take a bus home. While walking to the bus, Takagi and Nishikata agree to return to the water park together in the future.
In a short scene after the credits, Nishikata is seen sleeping on the bus ride home while leaning his head on Takagi, who is visibly flustered.