I am part way into my first ever playthrough of Oblivion and loving every minute of it. Bought Shivering Isle on my dinner break today and wanted to check before installing if I am safe to do so when already part way into the game. I am not bothered about mod compatability, just wanted to check with people in the know to make sure that installing SI now wouldn't damage my saves or anything major like that? My Oblivion is patched to the newest version, as well as running the unoffial patches. Will i need to re-patch after installing SI? Basically worried about losing 50+ hours of gameplay and wanted to check with people who have done this before.

Thanks.

No nothing will happen.
A quest leading you to the portal for entering the SI realm will automatically be given to your character after 24 ingame hours.
Again, nothing will happen to your current saves don't worry about it.

Thanks for the quick reply. Will go and install now.

Yeah, when you install the expansion, after a while you'll "hear a rumour about a door in Niben Bay". Then you're on your way to the Realm of Madness.