Question 1

Can I open an OST file without the Exchange or Microsoft 365 account?

Accepted Answer

Not in Outlook, which only opens an OST through the profile that made it. This page needs neither the account nor Outlook: it reads the file itself. To just read it, open it in the PST Viewer; to have it in Outlook again, convert it here and open the PST.

Question 2

Are contacts and calendar items in the OST converted too?

Accepted Answer

No, only the mail: the new PST is written with email properties, which would lose a contact's phone numbers or a meeting's times. The page says how many contacts, calendar items and tasks it leaves out. Open the OST in the PST Viewer to save the contacts as .vcf or .csv and the calendar as .ics, which Outlook imports.

Question 3

Why will Outlook not open my OST file?

Accepted Answer

An OST is tied to the Outlook profile and account it was made for. A new computer, a new profile, a removed account or a changed password all leave it orphaned, and Outlook says it cannot be opened or is in use. The mail is still in the file; this page copies it out into a PST, which any Outlook opens.

Question 4

Is anything in my mail changed or lost?

Accepted Answer

The OST is only read, never changed. In the PST each email keeps the fields listed above. Outlook's categories, flags for follow-up and rules are not carried over, and an email the reader cannot read is listed as left out rather than stopping the rest.

Question 5

Which OST files does it read?

Accepted Answer

OST files from Outlook 2003 and later, including the newer format Outlook 2013 and later use, read by the same open-source reader (pst-extractor) as the PST Viewer. If an OST will not open, try it in the PST Viewer first: if the viewer cannot read it either, it cannot be converted here.

Question 6

What if the mail is bigger than one PST?

Accepted Answer

One PST made here holds up to about 3.3 GB. When the mail of a bigger OST reaches that, the page finishes the first PST and offers a button to put the rest into a second one, and so on. Chrome and Edge write each PST straight to disk; other browsers build it in memory, which suits a gigabyte or so per file.

Convert OST to PST