Both are acceptable and clear but to my ears, "to be OK with someone" is more natural than "to be OK for someone".

都可以，but I agree with Adrian that "Is 6 o'clock OK with you?" is a little more natural.

Both "Is 6:00 OK with you?" and "Is 6:00 OK for you?" are good, natural, colloquial English.

They are slightly different. 'Is ....OK for you?' means 'Is this suitable/convenient for you?' This is likely to be about a fact. 'Is ....OK with you?' means 'Do you have any objections to this?' This is likely to be about an opinion. For example, if you are planning a dinner party, and you know that a guest has allergies and isn't able to eat certain foods, you would need to ask if a dish is suitable, so you would say 'Is this OK for you?' . You wouldn't say 'Is this OK with you?' in this situation, because this is about a fact, not an opinion. 'That's OK with me' - or even 'I'm OK with that' - means 'I don't mind'.