How to Recreate an OST File in Outlook

Your OST file is acting up. It is throwing sync errors, it has grown huge or it just will not load right. A common fix is to make Outlook build a brand new one. That is what recreating an OST means.

It is a clean fix when it fits, but it can also wipe data you cannot get back. This guide shows the safe way to recreate an OST file in Outlook, plus the one check to run before you delete a thing.

What does it mean to recreate an OST file?

Recreating an OST does not repair the old file. It throws it away and builds a new one. You close Outlook, delete or rename the .ost file, then reopen Outlook. With nothing to load, Outlook makes a fresh OST and downloads your mailbox from the server again.

This works because the OST is only a local copy. The real mail lives on the server, so a fresh download rebuilds the local copy from scratch. That clean rebuild is what clears out corruption and bloat in one move. It also means the whole trick depends on one thing: the server actually having your mail.

When should you recreate the OST?

Recreating is the right call when the local file is the problem, not the mail itself. A few clear cases point to it.

• Outlook keeps throwing sync errors that will not clear.
• The OST has grown huge and is slowing Outlook down.
• The file is mildly corrupt and a repair did not hold.
• A bad add-in or a disk fault left the file in a strange state.

In all of these, the mail on the server is fine. Only the local copy is broken or bloated, so a fresh download fixes it. That is the line to keep in mind, because it decides whether recreating is safe at all.

Before you delete anything: is it safe to recreate?

This is the step most guides skip. It is the one that matters most. Recreating deletes your local copy. If the server has everything, you lose nothing. If it does not, you lose whatever was only in the file.

Run this quick check before you delete the OST.

How to recreate the OST file in Outlook

Once you have confirmed the server has your mail, the steps are short. Back up the file first, every time, so a wrong guess does not cost you anything.

The default location, if you would rather browse to it yourself, is below.

C:\Users\<YourUsername>\AppData\Local\Microsoft\Outlook\

A lighter fix: compact the OST instead

If the only problem is size, you may not need a full rebuild. An OST does not shrink on its own. Deleted mail leaves empty space inside the file, so it keeps growing. Compacting reclaims that space without re-downloading anything.

Go to FileAccount SettingsData Files, pick the file, open Settings, then choose Compact Now. It is slower than it sounds on a big file, but it is far gentler than a rebuild and it carries no risk to your data. Try this before you delete the OST if bloat is the only complaint.

What actually happens when you recreate an OST

Most guides stop at “delete it and reopen Outlook.” That works, but it hides what the rebuild really does. A few of those details decide whether you end up happy or short a few years of mail. This part goes deeper. It is more technical, so it sits here at the end.

It is a full re-download, not a repair

A repair fixes the file you have. A rebuild does not touch the old file at all, it pulls a clean copy from the server from zero. Outlook normally syncs in small steps, tracking what it already has with change markers so it only fetches what is new. Delete the OST and those markers go with it. The next sync has nothing to compare against, so it downloads the whole mailbox again. On a small mailbox that is quick. On a 40 GB one it can run for hours and hammer your connection, which is worth knowing before you start it on a Monday morning.

Why the new file can be smaller than the old one

Here is a surprise that catches people out. Cached Exchange Mode only keeps mail for a set window, often the past 3, 6 or 12 months, set under the account’s sync slider. Your old OST may have quietly built up years of mail beyond that window over time. Rebuild it and Outlook only pulls back what fits the current window, so the new file can look like mail has gone missing. It has not. It is still on the server, just no longer cached on your PC. Widen the sync window and let it sync again to bring more of it back down. Microsoft explains the window in its guide to Cached Exchange Mode.

Why recreating is destructive in an investigation

From a forensic seat, recreate is a word to treat with care. The OST is often the last place a deleted or aged-off message still exists, because the local cache lags behind what the server has pruned. Delete the file to rebuild it and that history is gone in one click, with no copy left. So if a mailbox might be evidence or holds anything that is not also on a live server, you do not recreate it. You preserve it.

Does the new Outlook have an OST to recreate?

No. This changes the whole question. The new Outlook for Windows does not build a classic .ost file. It keeps its local copy in a different store under a folder named Olk, so there is no OST to delete and rebuild the old way. Everything above describes classic Outlook, which is still where almost every .ost file you will meet comes from. If you want the background on the file itself, see our explainer on what an OST file is used for.

Final word

So recreating an OST is a clean, simple fix when the server holds your mail. Close Outlook, set the old file aside and reopen. A fresh copy downloads in its place. That clears corruption and bloat without any fuss.

The whole risk sits in one question you answer before you delete anything. Does the server have a full copy of this mailbox or is the OST the only place some of it lives?