What Is an OAB File? The Offline Address Book Outlook Downloads From Exchange
An OAB file is your company address book, copied onto your computer so it works offline. Jump to your question below, or read it through in order.
An OAB file is your company address book, copied onto your computer so it works offline. Jump to your question below, or read it through in order.
An OAB file is an offline address book. Exchange builds a copy of your organization's address lists, and Outlook downloads it so you can look up a colleague by name with no connection to the server.
It is a cache, not a document. You never open one, and deleting it costs you nothing, because everything in it lives on the server as well.
Type three letters into the To box at work and Microsoft Outlook finishes the name for you. That happens even on a plane with the wifi off, and the OAB file is the reason.
Microsoft's own wording is short. An offline address book is "a local copy of an address list collection", used for address book lookups by Outlook clients running in cached Exchange mode.
Two words in that sentence do the work. Local, meaning it sits on your computer. Copy, meaning the real thing is somewhere else.
Everything else on this page follows from that. An OAB file is disposable. When it goes wrong the fix is almost never to repair it, it is to throw it away and let Outlook fetch a fresh one.
| File | Holds | Lives on | Safe to delete? |
|---|---|---|---|
.oab | The company address list | Your computer, as a cache | Yes. Outlook downloads it again |
.ost | A cached copy of your mailbox | Your computer, as a cache | Yes, though it takes longer to rebuild |
.pst | Mail, calendar and contacts | Your computer | No. Often the only copy there is |
.edb | Every mailbox on the server | The mail server | No. This is the original |
Read the last column and the pattern is clear. The OAB and the OST (Offline Storage Table) file are both caches of something the server already holds. A PST file format is not, and an EDB (Exchange database) file is the original everything else copies from.
No Exchange, no OAB. A plain POP or IMAP account has no offline address book at all, because there is no company directory behind it. If you cannot find one on your computer, this is usually why.
Microsoft Exchange Server and Exchange Online both produce one, so a Microsoft 365 mailbox has an offline address book in exactly the same way an on-premises one does.
Offline address book, offline GAL and Offline Global Address List are three names for the same thing. The Global Address List is the live directory on the server; the OAB is the copy you carry.
This trips people up constantly, and it is worth thirty seconds because most instructions you find online use one name while your screen shows another.
| Name you will see | What it means | Live or copy? |
|---|---|---|
| Global Address List, or GAL | The directory of everyone in the organization | Live, on the server |
| Offline Global Address List | The downloaded copy of that directory | Copy, on your computer |
| Offline Address Book, or OAB | Same downloaded copy. This is Microsoft's own term | Copy, on your computer |
| Address Book | The Outlook window that shows either one | Neither. It is the viewer |
That last row matters more than it looks. The Address Book window in Outlook has a dropdown at the top, and the entry you pick decides whether you are reading live data or your copy.
Pick Global Address List and Outlook asks the server, so a colleague who joined this morning is there. Pick Offline Global Address List and Outlook reads your cache, so they are not.
This is the quickest way to prove where a problem is. If somebody is missing from your address book, switch the dropdown to the online Global Address List and look again. If they appear, the server is fine and your copy is stale. Section 5 refreshes it.
The OAB holds names, addresses and the properties Outlook needs to autocomplete and resolve a recipient. It deliberately leaves out group membership, the organization hierarchy and some custom fields.
The offline copy is smaller than the live directory on purpose. Trimming it is what makes it small enough to ship to every laptop in the company every day.
Some custom properties are missing too. If your organization added a field such as employee ID to the directory, it may be in the live Global Address List and absent from your offline copy.
None of that is a fault. It is the trade the format exists to make: enough to address an email offline, not enough to browse the whole company.
Full details versus no details is an administrator choice, and it shows. Microsoft warns that a no-details offline address book lacks properties Outlook expects and "might make Outlook unresponsive at certain times".
If Outlook hangs when you open the address book on a laptop that is offline, this setting is worth checking before anything else.
Outlook keeps OAB files in AppData\Local\Microsoft\Outlook\Offline Address Books. Inside are GUID-named folders, one per Exchange account. The names tell you nothing about which is which.
Microsoft documents the parent folder, and that is enough to locate the files. The part that catches people out is what sits inside it.
Open that folder and you will not see a file called anything useful. You will see one or more folders named as a long string of letters and numbers, a GUID.
There is one per Exchange account on the computer. Somebody with a work account and a second mailbox has two, and nothing in the folder name says which is which.
Outlook will tell you, and this trick is worth knowing because the alternative is deleting the wrong account's cache.
If that field shows a web address starting https:// rather than a folder path, the address book has not finished downloading yet. Wait for it to land before you go hunting for files.
You can, though it is rarely worth it. The setting is a registry value, and there is a matching Group Policy for managed machines.
SpecifyOfflineAddressBookPath value of type REG_SZ, set to the folder you want.
Either way the mail profile has to be recreated before Outlook uses the new location, which is usually enough to make people leave it alone.
To refresh the address book: Send/Receive tab, Send/Receive Groups, Download Address Book. Then untick Download changes since last Send/Receive, or you get a partial update instead of a fresh copy.
Use this when somebody is missing from your address book, a name resolves to an old address or a colleague who left is still showing.
Step 4 is where most guides go wrong. They tell you to tick "Download changes since last Send/Receive", which asks the server for a differential update. That is the right setting for a routine refresh and the wrong one for troubleshooting. If your local copy is damaged, a differential update patches the damage rather than replacing it.
Untick it and Outlook downloads the whole address book again.
Older instructions say to find this under Tools. That menu was removed in Outlook 2010. A guide that sends you there predates 2010, and the rest of it is probably out of date too.
If a full download still leaves the address book wrong, the local files themselves are the problem and section 9 deletes them.
Exchange builds the OAB in a hidden system mailbox and publishes it to a web address. Outlook then fetches it over HTTPS. No part of that runs on your computer.
Three things happen in order. A failure at any one of them looks identical from your desk.
OABGeneratorAssistant builds the files. It runs inside a hidden arbitration mailbox, not on a server you can point at.Step two is the one worth remembering. Public folder distribution, which older guides still describe, was removed in Exchange 2013. Microsoft's wording is plain.
“Only web-based distribution is supported (public folder distribution is no longer available).”
So when an address book will not download, the question is rarely about the file. It is whether Autodiscover is answering and whether that web address is reachable.
Exchange rebuilds the OAB every 8 hours. Outlook downloads it on a separate schedule, and Microsoft's own two guides give different numbers for that: 24 hours in one, 48 in the other.
Almost every page on this subject gives a single number, usually 8 hours. That number is real and it is about the wrong half of the process.
| Clock | Default | What it controls |
|---|---|---|
| Server generation | Every 8 hours | How often Exchange rebuilds the files. A change made in Active Directory is not in the OAB until this runs |
| Client download | 24 or 48 hours | How often Outlook fetches the rebuilt copy. See below |
This is not a misreading. Two current Microsoft pages give two different figures for the same behavior.
The Exchange Server guide says Outlook clients are configured to download the OAB every 24 hours by default. The Outlook administration guide says Outlook in cached mode "automatically updates the offline address book on the client every 48 hours", counted from the last successful download.
We are not going to pretend to resolve it. What matters for you is the shape of the answer, and both numbers give the same shape.
Worst case, a directory change can take well over a day to reach a desk. Up to 8 hours waiting for Exchange to rebuild, then up to another 24 or 48 before Outlook asks for it. That is why a new starter is not in the address book on their first morning, and why nothing is broken when that happens.
Competitor pages that quote 8 hours as the client interval are reading the server number and attaching it to the wrong half. If you have been told the address book refreshes every 8 hours and it plainly has not, that is the reason.
Neither clock has to be waited out. A forced download ignores the client schedule, and an administrator can rebuild on the server without waiting for the next generation.
You cannot open an OAB file, and there is nothing in it you would want. It is a compressed cache Outlook reads for you, not a document.
Search this and you will find pages explaining how to open one, usually just before offering to sell you something. The straight answer is shorter.
The reason people go looking is almost always one of two things, and neither needs the file opened.
Open the Address Book with Ctrl, Shift and B and pick the Offline Global Address List. That is the file's contents, rendered properly.
Export from the live directory, not from the cache. An administrator can pull the Global Address List from Exchange, which is complete and current.
An address book that is out of date, missing names or throwing 0x8004010F is almost always fixed by making Outlook download a fresh copy. Deleting the cache is safe, because the server holds the original.
An address book that is not updating is the most common complaint, and a corrupt local cache is the most common cause. Every step here is safe and reversible, because there is nothing in an OAB file that is not also on the server.
Offline Address Books, using the Properties trick to pick the right one.Step 4 sounds drastic and is not. You are deleting a cache. The worst case is that Outlook spends a few minutes downloading the address book again, which is exactly what you wanted it to do.
This is the error people meet most often with the offline address book, and the wording is unhelpful: the operation failed, or an object cannot be found.
What it means is that Outlook could not reach or write the offline address book. Three things cause it.
| Cause | What to do |
|---|---|
| A damaged local OAB folder | Delete the GUID folder and let Outlook download again. Steps 4 and 5 above |
| A broken Outlook profile | Create a new mail profile and test with it. Step 6 above |
| Autodiscover not answering | An administrator job. Outlook cannot find the web address the OAB is published to |
The same code turns up in other contexts, which is why searching it returns so much noise. Our guide to 0x8004010F after importing an MBOX file covers the profile side of it in detail, and the profile fix is the same one.
Ignore any fix that starts with Internet Explorer. A widely copied 0x8004010F procedure walks you through LAN proxy settings in Internet Explorer for an Exchange 2007 server. Internet Explorer is retired and Exchange 2007 went out of support in 2017.
That page is still near the top of the results. It will not help and the settings it changes affect the whole machine.
On the Exchange side the address book is managed with the OfflineAddressBook cmdlets. To rebuild one now, run Update-OfflineAddressBook.
Use this when a directory change needs to reach users before the next scheduled generation, or you need to see which address book a mailbox is actually assigned.
These run in the Exchange Management Shell, which is PowerShell with the Exchange cmdlets loaded. Creating a new address book uses New-OfflineAddressBook, though most organizations never need more than the default one.
Update-GlobalAddressList is not the command you want here. It appears in more than one popular guide as the way to refresh an offline address book. It is a different cmdlet for a different object, and running it will not change what your users download.
The one that rebuilds the offline copy is Update-OfflineAddressBook.
Rebuilding is not instant either. The generation runs, then each client still has to ask for the result on its own schedule, so allow for both clocks before deciding it did not work.
Only OAB version 4 is supported. Microsoft publishes the full file format as an open specification, [MS-OXOAB], which is unusual for an Outlook format.
You do not need this section to fix anything. It is here because two details explain behavior that otherwise looks arbitrary.
.oab[MS-OXOAB], Offline Address Book File Format and SchemaOABGeneratorAssistant on ExchangeMicrosoft's wording is blunt: only OAB version 4 is supported. It is Unicode, and it allows clients to receive differential updates instead of always downloading everything.
That second property is the reason the differential checkbox in the download dialog exists at all. Version 4 made partial updates possible, which is what keeps a daily address book download from being a daily full download.
Versions 1 to 3 were ANSI-era and were dropped in Exchange 2013. If you find guidance about picking an OAB version, it predates that release.
The format is documented in [MS-OXOAB], one of the Exchange Server open specifications. It describes versions 2, 3 and 4. It is also still maintained, with the current revision at 16.0, dated May 2025.
That is worth noting against its neighbours. Microsoft has published a full specification for Outlook Data File (.pst) and for the offline address book, and has never published one for the OST file format. Three Outlook files, three different levels of openness.
Two questions sort out most of what is left. Answer them in order.
Is the name there online? Switch the Address Book dropdown to the live Global Address List. If the person is missing there too, this is not an OAB problem at all.
Have you actually replaced it? A download with the differential box ticked is not a fresh copy. Section 5 has the one setting that decides this.
Seeing a numbered error instead? Our Outlook PST error fixes cover the data file side, and the New Outlook errors guides deal with the rebuilt client.
Working out which Outlook file is which? The OST offline cache guide covers the other copy on your computer, and EDB file format explains the server database it all comes from.
Found something here that does not match your version? Send us the exact wording on screen and we will check it against Microsoft's docs.