in a tone of conversation they can mean different things. saying "OK" with both letters capitalized is putting emphasis in such a way that you'd say it to someone you were annoyed with who was telling you to do something many times. "o.k." is how you would answer the question "how are you?" though the periods aren't necessary. and fully typing out "okay" is what you would use at the end of a sentence to see if it is alright ex. "are you okay?" or "is this okay