if that's OK with you とはどういう意味ですか?

質問を翻訳

"I'll borrow your van keys if that's OK with you."
It is a question in the form of a statement
It means "May I borrow your van keys?"

The way I remember it, people kind of stopped saying
"Is that OK with you?" and started saying "Are you OK with that?" in the 1980's maybe? I am not sure. But really you can say either one.

Almost always it is "with" but you can say something like
"I can schedule a 3:00 appointment if that is OK for you."
I think the reason that sounds natural is because it is an appointment "for" you.

In that specific case, however, I think it would be more natural to say "I can schedule a 3:00 appointment if that works for you."