Simple phrase. Three words.
Weight of each indicidually is so nil
Together they are powerful
When you hear them, you may not know what else is going on in the mind of the one who says it
Sometimes, its just a way to not let small talk take place
Sometimes, it may be the end of a long fight
Sometimes, it may be the 3rd response in an IM chat. A response that triggers so many questions in the mind of one who reads it
Sometimes it also depicts a state of mind, that makes other perceive you as disinterested
Sometimes, its a motivator for oneself to lift up from a state of dejection.
Sometimes, it just depicts that you are not interested in any further discussions or you just don't wanna listen to any excused anymore.
You may wanna ponder that when on phone, you say "I'm OK" to a friend who calls you after a long time, you may be hiding something either from yourself or the person on phone
If you stress more on "am", it may describe some sort of frustration inside.
Stressing on "OK" may signify that you are just not concerned at all about any thing or least interested in listening to the other party.