OST or PST, One Format Wearing Two Contracts

Both files are written to one Microsoft specification. Line them up byte for byte and you are looking at the same structure twice. What separates them is not built into the file at all. It is the terms Outlook attaches to each. Those terms decide what you can do on the day something goes wrong.

Knowing what each file is for answers almost nothing when you are standing in front of a machine that will not open one. What answers it is knowing which of the two you are holding, a PST or an OST, because in six ordinary situations that alone decides the outcome.

One Specification, Two Sets of Terms

Personal Storage Table and Offline Storage Table. Two names that sound like two formats, and Microsoft publishes a single one for both. The Wikipedia record puts it plainly, describing the personal store and noting that the same format acting as a cache for Cached Exchange Mode simply takes a different extension.

Two identical structures, then, behaving nothing alike.

The repair tool is where that identity gets argued about. Plenty of guides state that the Inbox Repair Tool, ScanPST.exe, fixes both. Others say flatly that it does not and that no repair tool for a cache has existed since ScanOST was dropped after Outlook 2010. Microsoft documents the tool for personal stores and does not claim the other, so the honest position is that this one is unsettled. For a cache it rarely matters, because the supported fix was never repair. It was to delete the file and let it rebuild.

A PST is given. An OST is lent. That is the shortest way to hold the distinction. A personal store is handed over outright and stays yours whatever happens to the account it came from. A cache is lent for as long as the profile and the mailbox both survive. The loan ends without notice.

Six Situations Where It Decides the Outcome

Each of these is ordinary. Every one turns on which file somebody happens to be holding.

Every entry in the right hand column depends on something outside the file still being alive. Not one entry on the left does.

The fourth row is the one that costs people time. Copying an OST to a new machine is the natural instinct and it is wasted effort. Set the account up again and Outlook builds a fresh cache from the server, which is faster than any copy and gives you a file that actually works.

What Outlook Checks Before It Reads Anything

Renaming an OST so that it ends in .pst is the idea everybody has. It fails, and the reason is an ordering that nobody would guess.

open file
recognise the layout
read the folders
show the mail

open file
check which profile owns it
  no match, stop here
  the rest never runs

What Each One Actually Costs You

Neither is free. They charge you in different currencies. Knowing which bill is coming matters more than knowing which file is better.

That row surprises people. Outlook offers a password on a personal store and offers none at all on a cache, which reads as though one is protected and the other is not. Neither is. That password stops nothing. The encoding inside a cache is not encryption in any useful sense either. Real protection is disk encryption, which is a separate decision from either format.

Which One You Have, Without Opening Anything

Two minutes with the Account Settings dialogue answers this. The answer shapes everything else.

1. Open the Data Files tab. File then Account Settings then Account Settings, then Data Files. Every store the profile knows about is listed with its path.
2. Read the extensions. Anything ending in .pst is yours outright. Anything ending in .ost is a cache and belongs to the account beside it.
3. Note where each one sits. Personal stores tend to live in a Documents folder because somebody chose to put them there. Caches sit in application data because nobody chose anything.
4. Check the account type in the list. Exchange, Microsoft 365 and, since Outlook 2013, IMAP accounts all produce a cache. Only POP accounts still produce a personal store.

Both in one profile is normal. A work account gives you a cache while an archive you made two years ago sits beside it as a personal store. Neither one is a mistake and neither replaces the other.

When the Server Is Also Gone

Everything above assumes the mailbox still exists somewhere. When it does not, the comparison changes shape entirely.

A personal store carries on as if nothing happened, because it never depended on a server. A cache becomes a file holding real mail that Outlook will not open, which is the worst of both positions and the reason most people ever look this up.

PST  attach it and read
OST  sign in, it rebuilds

PST  attach it, read it
OST  Outlook refuses
     the mail is still inside

At that point the question stops being which format is better and becomes what can still be read. The mail is intact. What is missing is the account that granted access. No setting brings that back. Where the mailbox lived in an Exchange EDB that has also been decommissioned, the local file may be the last copy in existence.

Reading one in that state is a parsing job rather than a mail job. Our OST viewer does it in a browser tab with no profile and no Outlook involved. Where the contents then need to live somewhere current, the converters handle the route out.