Start a casual conversation with the any random person on the street, and ask them how is it going? You will get the generic, vanilla answer, O.K.!
When did we become such a homogenized society that the complexity of emotions we experience can be summed up in the neat and tidy phrase O.K.? O.K. is a pair of initials with a multitude of uses -- not to mention a number of different explanations of how it came to be.
Based on information from the trusty website Wikipedia, it states that OK has multiple usages and as many theories of origin. As an adjective "OK" means "adequate", "acceptable" ("this is OK to send out"), "mediocre" often in contrast to "good" ("the food was OK"); it also functions as an adverb in this sense.