No, it is not. Words with Friends uses the Enhanced North American Benchmark dictionary, which does not include "OST" as a word. Scrabbles uses the SOWPODS dictionary, and similarly does not count "OST" as a word, though "SOT" is. You can use free word unscramblers like wordunscrambler.co to enter your own letters and see what counts for each game.
In Sindarin, the word for castle or fortress is ost.
East = Ost East = Osten
OST Official Sound Track
"Ost" can refer to different things depending on the context. In music, it commonly stands for "original soundtrack," which features the score of a film, game, or show. In another context, "ost" is a German word meaning "east." Additionally, it can be an abbreviation for "Office of Science and Technology" in governmental or academic settings.
Nord-Ost was created in 2001.
Saint-Ost's population is 81.
Ober Ost ended in 1919.
Ober Ost was created in 1915.
Daniel Ost was born in 1955.
Warren Ost died in 1997.
Ayzac-Ost's population is 412.