I know of two, but don't remember the name of the other. One is called WebEx, and it's a pay service. It's excellent. You can tell your dad to relinquish access to his desktop, and then you can control it, and do everything as if you're right on it.
I don't remember the name of the other, but it was Java-based, and many years ago.
I have no idea if either of these will work with Mac, anyway.
There's also a way to do it with NetMeeting, if you've got M$ Office. But you said OSX.
I'm sure there are some free ways of doing it, too -- see java desktop sharing.