Oshimaen

Oshimaen is an amusement park in Onimai. Mahiro Oyama and Mihari Oyama visited the park as children, and later visited it again many years later as Mahiro's birthday present to Mihari.

History
Mahiro and Mihari visited the park with their mother as children when Mihari was seven years old. During the visit, Mahiro won Mihari a stuffed "Oshima" plush toy and gave it to her, which made her extremely happy. Later in the day, she got separated from them and started to cry until Mahiro found her again.

In "Mahiro and the Gifted Experience", Mahiro decides to bring Mihari back to the park as a birthday present after a suggestion from Kaede. There, they spend the day enjoying the rides and other events, though they note the lack of attendees and suspect that the park has fallen on hard times. Mahiro even plays the same shooting game and wins Mihari another (albeit different) "Oshima" plush toy. When they are exhausted, Mahiro notices an arcade on the park grounds. While inside, she pays for a photobooth game that the two of them can use. Afterwards, Mihari cries tears of happiness at the picture, noting that it's not fair, as one can't buy such happy memories.

In "Mihari and the Stuffed Animal", Mihari thinks about the first time they visited the park, and notes that Mahiro's good attitude towards her hasn't changed even after all these years.

Trivia

 * As noted in "Mahiro and the Gifted Experience", the name of the park is taken from the Japanese title of the series Onii-chan wa Oshima i!, with "-en" added onto it, which means "park".

Manga

 * Mahiro and the Gifted Experience
 * Mihari and the Stuffed Animal