Are you running windows on the mac? If so, there is a program called iTunes Library Updater that will refresh your library (i.e. add any new songs you've added/bought from iTunes while running OSX) to your Windows version of iTunes. If not, and Windows is running on a separate machine to your mac, then if you enable home sharing, you can set your libraries to sync with each other. To do this, enable home sharing, and your mac will show up in the left hand pane of iTunes. Select it and at the bottom there is a 'Settings' button. Select this and you can choose whether to sync music, podcasts, apps, etc. Just do this for both your mac and your windows machine.
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