Thank you for your sincere welcome and quick reply. Do you mean okay, instead of ok, is the only right spelling form? And I take your advice and I will only use you're welcome in response to thank you from now.Welcome to the Forum, Yuansheng!
Please note that "ok" is not a word. If it were a word, it would be pronounced like "oak."
What you mean is written either as heypresto has written it, OK, or spelled out, okay.
As mentioned in other threads like this one here, there are many casual and informal alternatives to "you're welcome."
Please keep in mind that a lot of people find some of the casual alternatives annoying and even rude.
I'd recommend sticking to the always polite "you're welcome" in response to "thank you."