Looked through the OIS implementation for windows and figured it out myself.
For anyone else looking for the answer:
At input system creation, apart from the "WINDOW" parameter, need to pass these on too, in the ParamList
pl.insert(std::make_pair(std::string("w32_keyboard"), std::string("DISCL_BACKGROUND" )));
pl.insert(std::make_pair(std::string("w32_keyboard"), std::string("DISCL_NONEXCLUSIVE")));
To the OIS implementers:
Great library guys, I'm using it and I love it!
One small note though: please document the available parameters. Also, the NONEXCLUSIVE mode should perhaps be the default, otherwise the end-user would have a very annoying experience with the resulting application using OIS.
Have a nice day!