Updated: Apr 1, 2020
Original soundtrack or OST is usually the instrumental pieces composed especially for the movie, while soundtrack refers to all the music in a movie.
The soundtrack can be a cover song or a recreation of a classical piece, it is literally the track of sound (music in this case) that runs with the movie. An original score that sometimes also is called OST is the original music composed for that film.
Even when the movies were silent, there was a piano player accompanying the silence black and white movie live in the cinema. By our definition, even that music should be considered as a soundtrack. And if the piano player would play some original works of his own, or started to accompany the movie with improvisation, that should be considered as OST or original score.
Time passed and a “soundtrack” was added to the movie rail. Now we could hear the voice of actors and actresses, the sound effect of whatever was happening on the screen and pre-recorded music accompanying all of them. So the music was being played from a track beside the pictures in the movie rail, thus we had a soundtrack especially for music. And the soundtrack came to existence.
Many great classical composers like Aaron Copland and Sergei Prokofiev composed original music for the movies. And many great composers like Ennio Morricone never composed anything except for the movies.
In the twentieth century that was the age of popular music, cinema saved classical music from extinction. Many classical pieces are recreated and re-performed for movies. Beethoven's 7th symphony has been in a lot of movies. Chopin’s music was revived several times by movies. All of these are soundtracks. There are also a lot of original scores in the same genre being composed of the movies. The OST of the trilogy “Lord of The Rings” can be considered a very serious classical composition as well as a very brilliant original soundtrack.
We can also refer to a certain type of music as the genre of the soundtrack. That is a different subject than what we had studied here.