I do hope a lot of the comments in here are sarcastic, and everyone is aware that Oy. is the Finnish equivalent of Ltd. or LLC.
http://en.wikipedia.org/wiki/Osakeyhtiö
So basically they will rename it Microsoft Mobile, seems reasonable. MS can't continue calling the company Nokia, as that will be retained by the remainder of Nokia (the NSN network infrastructure, HERE maps and services, patent licensing, etc.) though MS has a license to sell phones under the Nokia brand for a limited time.