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.
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OST Official Sound Track
cost host lost most post
quix is an acceptable dictionary word for games like scrabble, words with friends, crossword, etc
There are 2 blanks In words with friends
There are 5 S's in Words With Friends. (The prior answer noted that there were 5 S's also in Scrabble. This is incorrect. In Scrabble, there are only 4 S's; Words With Friends features an additional S in its letter distribution.)