The difference between the OST and the theme song was the scope and application. OST referred to all the original soundtracks in movies, TV series, anime, or games, including theme songs, interlude songs, and ending songs. The theme song was only the opening song of the work. The theme song was a complete song, used to represent the main piece or song of the work. Therefore, it could be said that OST was a part of the theme song, but OST was not the same as the theme song.
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.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.