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
Daniel Ost was born in 1955.
Ayzac-Ost's population is 412.
Warren Ost died in 1997.
Can This Be Love OST was created in 2005.
Ober Ost ended in 1919.
Ober Ost was created in 1915.
Nord-Ost was created in 2001.
Saint-Ost's population is 81.