You have to dive a little bit into the sf::Window class, and then into the sf::WindowImpl class. But it should be very easy to get the OS-specific window identifier. Each sf::WindowImpl derived class (one for each OS) stores its window id, so you should be able to return it with a virtual function. There is even a portable type for it : sf::WindowHandle.
But, why do you need OIS ? If something in OIS is better than in SFML I should really think about adding it.
Thanks for the quick answer, you must have posted while I was writing my post

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Well.. the main reason is that i've been using OIS before and it's got all possible functionality i've ever needed besides for rumble for Xbox360 controllers but if i recall right only XInput handles that at the moment and that's probably not a concidence heh

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At first i didn't dig deep enough in SFML to notice that you're only missing a few defines to be able to use all keys either so i guess i could go with SFML's sf::Input too by only adding a few defs (i need all the keys on the keyboard for my network chat, the SHIFT keys for example) but now i've got OIS working anyways ^^.
Please add all the possible keys to:
Key::Code WindowImplWin32::VirtualKeyCodeToSF(WPARAM VirtualKey)
If you've got time

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Great design on the lib btw!