OST file size can be reduced using the In-built utility in Outlook. Reducing file size might help in improving Outlook performance provided that the user has deleted a number of items. To compact OST file,
Below are some steps to follow:
Delete all those items which are not required.
Open folder list view and in the Navigation pane, right click on the folder, click Deleted items and click Empty folder.
In Outlook, Go to the File menu -> Account Settings, select Account Settings.
Click on the Data Files tab, select the data file to be compacted and click Settings.
In the opened dialogue box, click the Advanced tab and then click Outlook Data File Settings.
In Outlook Data File Settings dialogue box, click Compact Now, which is the final step.
Maybe your problem is resolved