Is it considered rude when i say "OK" when someone says thank you?

Depends how you say it and who to. If it's people you're familiar with, it's not a problem. If it's strangers in a formal setting then it's more appropriate to say "you're welcome".

I'm happy to say thank you.. I'm not really looking for a reply!
I think it's fine.

Say "you're welcomed." "Ok" doesn't express enough.

It's not really the appropriate response

yeah. the right thing to say is you're welcome

Its not rude but unnecessary doest seem the right response. I usually just smile or say "no problem" depends on the person & situation.

Depends on if you actually care about the answer.

ok...

Not really. I've seen ruder.

You already know this
I would take OK as " So what Bitch, now run along"

Yes its weird.