You can make 174 valid words from the letters in IMOPRST using the Scrabble US/CAN dictionary.
How many points is IMOPRST worth in Scrabble?
IMOPRST is worth 11 points in Scrabble (US/CAN) and 13 points in Words With Friends.
How do I use the Word Unscrambler tool?
Enter any letters into the input box and press Search. The tool will return every valid English word that can be made from those letters using the dictionary you select.