Yes it is little hard to convert Outlook OST to PST file but using third party tool like ATS OST to PST converter you can perfectly access your lost Outlook OST data and you can convert any size of files because there is no file size limitation, It supports Microsoft Outlook 2016, 2013 and all below versions and the preview option which allows you to see the results before final conversion, you can take advantage of its cloud based application like Office 365 & Live Exchange Server.
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