Ost extension is related to Outlook Offline Folder File. You have chance to open ost files with Microsoft Outlook.

OST files stand for e-mail archives which saved by Microsoft Outlook software. Ost files are used for saving emails such as pst files. However, it can keep big size files too. When you give "use offline folder" command on Outlook software, ost files are produced automatically. 

If you don`t need your old e-mails you can always erase this file.


What to use ost

Microsoft Outlook

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