No matter your native tongue, the English word "OK" is almost universally understood. The same is true of "hello," "hi" and with "goodbye" or "bye." These are the most commonly used words in English-speaking countries and are widely understood worldwide.
Here's a question: Why do non-native speakers grasp the concepts of "OK," "hello" and "goodbye," but people who speak English as their first language, people who have gone to school since they were very young to learn to speak it properly, have no idea what these words mean?
An example: Suppose someone asks you how you're feeling, and you reply with "OK." Has it ever happened to you that a person says, "OK is not an answer." Don't you just want to slap someone when they say that? No, you don't? Well, me neither. That would be wrong, wouldn't it?
Anyway, has that response to your perfectly fine answer of "OK" ever ruffled your feathers, so to speak? (I assume, of course, that your feathers are strictly an illustrative device, much like the buttons that so many people are trying to push all the time, or the pot that some people just cannot help stirring.)
Of course it has, and do you know why? It's because you know "OK" is an answer. Any response at all, even if you were to give a non-sequitur, like "cheese flannels," would be an answer. Absolute silence is an answer. In fact, silence can be a more powerful response than speech. "OK" is perfect in its simplicity. "OK" can never mean anything more or less than what it does, which is neither bad nor good but between the two. It's the only word that can best be defined by using itself as a definition.
If someone tells you "OK" is not a sufficient response, you can go ahead and never listen to anything else they say ever again. They are one of the few people in the world who don't know what "OK" means. Feel free to ignore them until they leave and then excise them from your life as you would a weeping boil or a house fly, whichever image you feel more comfortable with.
"Hello" and "hi" are probably the most commonly used words in everyday life. Think of how often you've said either one just today. I haven't even left the house yet and I'm already at three that I can remember. There is nothing wrong with the words themselves. They've been doing just fine for the English language for however long they've been used (you can tell how thoroughly I've researched this topic). The problem comes from how some people react to them. If, for example, I say "hello" or "hi" to as a greeting, but for whatever reason you (the use of you is for purely illustrative purposes; I would never intentionally disparage you, wonderful you) respond with a head nod, what am I to make of that? If we were merely going to exchange nods, I really wish I would've been told ahead of time, before I wasted my energy allowing "hello" to pass from my lips.
"Goodbye" is also commonly misunderstood, especially in regard to what is expected when someone says it. Have you ever been leaving somewhere, and you say "goodbye" to everyone you feel like saying it to, and you get to the car only to realize you've forgotten something you need? Now you have to go back and get it, and you hope you don't see anyone you've already said goodbye to as a queer feeling of embarrassment comes over you. That's because you know "goodbye" means "until I see you again" or "see you later," but the unsaid meaning is "see you tomorrow." So, you have to go back in and get, let's say, your car keys or your best hunting knife or whatever it is you left at work, and you do come across someone to whom you already said "goodbye." What is the proper protocol? Do you say it again, or do you give a slight ashamed smile and say something like, "Forgot my birth control on the way out," even if you're a guy?
I'm not sure, actually. I don't have all the answers, and I really wish you would just leave me alone sometimes.
I mean, you're constantly bothering me with these requests. Anyway, goodbye.