"Stay Frosty Oscar Mike" is a cobmination of two phrases used in the US military, according to call of duty: modern warfare 2.
"stay frosty" means star cool; stay on your toes. "oscar mike" means on the move. together, they mean: "Stay alert while on the move!"
General: RAMIREZ! take out that battleship with your flashbang!
General: Stay Frosty Oscar Mike!