From [this glossary](http://www.screenwriting.info/glossary.php) for abbreviation used in movie scripts,

> **EXT.** => Outdoors

> **INT.** => Indoors

> **O.S.** => Abbreviation for Off Screen, denoting that the speaker is not resident within the scene.

> **V.O.** => Abbreviation for Voice Over, denoting that the speaker is narrating the action onscreen.

You can find more abbreviations on the linked page and [this page](https://freelance101.files.wordpress.com/2008/10/script-abbreviations.pdf)