"It's OK" and That's OK" - Sometimes I wonder which one to use.
"It's OK" and That's OK" . I believe there's a difference between them and you use then in different situations.

When someone makes a mistake and try to comfort her: I would say "It's OK" ??
When someone spills water on me, : I would say "That's OK" ??

Can you give me some examples with explanations?

Question

"It's OK" and That's OK" - Sometimes I wonder which one to use.
"It's OK" and That's OK" .  I believe there's a difference between them and you use then in different situations.  

When someone makes a mistake and try to comfort her: I would say "It's OK" ??
When someone spills water on me, : I would say "That's OK" ??

Can you give me some examples with explanations?

Answer

You can use either expression in both expressions. The difference:
It's OK. = いいよ。
That's OK. = それ、いいよ。

Not a lot of difference, really.

Answer

Yes, you can use either.

If you really want to pick on the difference, "that" implies some distance between you and the object/situation."It's OK" is fairly neutral. "That's OK" is like saying, "it's 'over there', it doesn't affect me personally."

Importantly, this is NOT a physical distance; it's the perceived distance in your relationship to the object or situation.

If you really wanted to split hairs. ;)

Answer

You are right. Both expressions can be used in spoken English.

Answer

Honestly, you really can use one or the other, neither one needs to be used for a specific situation.

If someone bumps into me on the street and says they're sorry, I'd say "Oh, that's okay".

If someone can't go to a movie with me, I'd say "Oh, it's okay, don't worry about it."

But you could seriously use either one, I never really think about it.