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Once you delete the emails that you no longer need as well as minimizing the emails that are brought into your OST file through synchronization settings, you can consider compacting your OST file.

This can be done by selecting File | Data File Management... from the menu,

Find your exchange connection that has your OST file in the list of data files on the Data Files tab. Double click the file or select it and click the "Settings..." button.

Switch to the Advanced tab and click the "Offline Folder File Settings..." button. This will bring up a dialog that has a "Compact Now" button on it. Click it.

Compacting your offline folder can take some time, especially if the file is large and you have removed a substantial number of emails from your mailbox or made significant changes to your synchronization settings. I recommend clicking it before you leave work for the evening. 

Alternatively, you can also disable the offline caching mode, then ensure that the OST file is deleted, then re-enable the caching mode. This will build a new OST file with your current synchronization settings. If you find that the above compact mechanism is taking too long, you might want to try this approach.