Refers to the main song of a movie, TV show, anime, game, or similar production. Often used for both opening and ending themes.
このアニメの主題歌はとても人気がある。
The theme song of this anime is very popular.
The theme song of this anime is very popular.
映画の主題歌を有名な歌手が歌っている。
A famous singer performs the movie's theme song.
A famous singer performed the movie's theme song.
That band is providing the theme song for this drama.
The theme song played at the end of the movie.
主題歌 is the general term for a show's theme song (opening or ending), while ＯＰ refers specifically to the opening theme.
挿入歌 is a song inserted at a specific scene within a show, whereas 主題歌 is the main opening or ending theme song of the whole work.
エンディングテーマ refers specifically to the closing theme, while 主題歌 can mean either the opening or ending theme.
Compound of 主題 (theme, subject) and 歌 (song).