The answer is: "OST song" refers to the song in the OST. OST referred to the original music in movies, TV series, and other film and television works, and the song part was the OST song. According to the search results, some OSTs of movies and television works included some songs, such as the theme song of the TV series OST of " A Thought of Passing Mountains," many of Zhou Shen's OST songs, and Yu Kewei's Top Ten Classic OSTs. Therefore, an OST song referred to the song part of the original music in a film or television work.
We should avoid focusing on the 'lemon' aspect as it may involve improper content. For regular Kancolle - Ushio fanfics, one characteristic could be the blending of different story elements. Kancolle has its own lore with shipgirls, and when Ushio is introduced, the fanfic might explore how his character meshes with this world. It could also play with different genres, such as adventure or mystery, while still staying true to the core identities of the characters from both the Kancolle and Ushio concepts.
The OST lineup for the second season of Celebrating Years had already been announced. Among them, Li Jian sang the theme song " In My Lifetime," Zhou Shen sang the ending song " Excuse me," Liang Long, Liu Duanduan, Zhang Haowei, and Guo Zifan performed the promotional song " Aiya." These songs would bring more musical elements to the second season of " Celebrating Years," adding more emotions and atmosphere to the plot.
One common theme is friendship. Ushio and Tora have a very special bond, and many fanfictions explore how this friendship grows and endures through different situations. Another theme is adventure. They often go on various quests in the original work, and fanfictions expand on these adventures.