Refers to the main song of a movie, TV show, anime, game, or similar production. Often used for both opening and ending themes.
The theme song of this anime is very popular.
The theme song of this anime is very popular.
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.
Compound of 主題 (theme, subject) and 歌 (song).