I'm not entirely sure as I haven't read it in great detail. But generally, it might be about the characters in 'boogiepop' engaging with light novels in some way, perhaps reading them, discussing them, or having adventures related to the content of those light novels.
Boogiepop is an important character. He/She (depending on the interpretation) is central to the story. Another one could be the characters who are directly involved with Boogiepop's actions and missions.
Well, in 'boogiepop and others light novel', the themes of mystery and the unknown play a significant role. The plot is filled with elements that keep readers guessing. Also, friendship and the power of human connection are likely themes. Characters may rely on their relationships with others to face the strange and often dangerous situations in the story.
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.
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.