It seems that it would be possible (I don't use either hotmail or OE to try it, but...)
click contacts when logged into hotmail, then click import on the left at the bottom, and you will get this screen (attached)
that then takes you to a page where you can upload a csv file (which you export from your OE address book) and that should be it.
Hi biman, what I have done as a temporary measure for him, is to put a Folder on his Hotmail Favourites, and copied his Address Book into it. He can open that online, and select which address he wants to e-mail to, then copy and paste it into the e-mail. Tt means he has access to it even away from his own computer.
I had looked at copying each one, but he has lots!