PST stores email locally; OST syncs with Exchange server.

PST stores email on the computer. OST keeps a local copy, but it stays tied to Exchange and syncs back when the server is there. That is the plain answer, and it is the one that matters when a mailbox move gets messy.

I keep coming back to that split because it explains most of the confusion. A PST file is a local data file. Outlook can use it to hold mail, contacts, calendar items, tasks, notes, and other Outlook data on the machine itself. It is often used for archives or older mail that someone wants to keep close at hand.

An OST file works in a different way. It is an offline file for accounts that connect to Exchange and similar Microsoft hosted accounts. Outlook keeps a copy of the mailbox on the computer, but the real work happens with the server. When Outlook connects again, the OST syncs changes in both directions.

That is why the words sound close and the behavior is not. A PST is a storage file. An OST is a sync file. One can stand alone. The other depends on the server side of the account.

For day to day use, the most important fact is simple. If the account is Exchange based, the mailbox data is still on the server, and the OST mirrors it for offline use. If the data is in a PST, it lives locally in that file. That difference changes how people back up mail, move it to another computer, and recover after trouble.

This is where a quiet little trap shows up. Many users think any Outlook file can be copied anywhere and opened later with no fuss. That is true for some PST use cases, but not for OST. Outlook can recreate an OST when needed for Exchange accounts, because the server remains the source. A PST does not have that same safety net. If it is damaged or missing, the local file is the thing people depend on.

I treat that as the core limit of the format. An OST is not a portable mailbox in the same way a PST is. It is a cached copy that depends on the account and its connection settings. That means the file alone does not tell the whole story. The mailbox type, account setup, and sync state all matter.

There is one more practical point worth keeping in view. Outlook setups vary. Microsoft changes features, names, and account behavior over time, and not every version or tenant acts the same way. So the rule is steady, but the details around it can shift.

That is why I like the clean version of the answer. PST stores email locally. OST syncs with Exchange server. If a mailbox problem starts with that fact, the rest becomes easier to sort. The file is either the whole local store, or it is the offline copy of a server mailbox. That is the line worth drawing first.

And that is the kind of line The Migration Brief tries to keep clear: one practical fix, one file format explained, and one safer way to move your mail.