Please, let me make it clear that my question is not asking how native speakers usually respond to "Thank you". Before posting this question I did some research and I also read this discussion: How do native English speakers respond to "Thank you"? In my opinion none of the suggested answers apply to my question.
This is a specific question on the appropriateness of two specific responses.
Let's take a situation when I ask my colleague to briefly explain me something. And after he has done it, I say, "Thank you".
Is it polite to respond with an "OK" to my "Thank you"? (That's how a native English speaker replied to me today.)
Is it acceptable in an informal situation to respond with "You welcome" (without "are") or even with "Welcome"? Responding, "You are welcome" sometimes feels too long to me :)
UPDATE. Today two different people responded with "OK" to my "Thank you" and "Thanks", so it seems it's not that rare.