Does "is that ok for you?" means the same of "does that work for you?"

Do they mean exactly the same? Is one form more formal/casual than other?

Can I say one of them in a email that is not very formal?

I think it depends on the audience, but I think, "Does that work for you?" sounds slightly more informal. However, my first reaction to either of those would not be that they sound "informal". Personally, I'd probably use, "Does that sound OK to you?"

Both of those options sound somewhat informal.

The more formal version of that phrase would be, "Does that sound acceptable?" or "Does that sound all right to you?"

"Does that work for you" is more commonly seen in the form of a response, not a question. As in, "that works for me."