Outlook 365 contacts and calendars don't synch ost and pst issues

Question

Outlook 365 contacts and calendars don't synch ost and pst issues

Drewskii 0

The Problem

Outlook 365 mail operates properly.

Contacts and calendar don’t synchronize.

The Outlook data folder shows evidence of multiple incomplete migrations and upgrades over the years — likely from an old Outlook version to Outlook 365, and possibly a failed "New Outlook" migration. This has left behind duplicate and conflicting data files that are confusing Outlook and preventing iCloud from registering properly.

The Outlook data folder contains multiple duplicate .ost files for the same accounts, evidence of a failed New Outlook migration, and duplicate .pst files — suggesting multiple incomplete migrations that are preventing iCloud from registering a live data file correctly.

The Smoking Gun

******@icloud.com.ost is only 16 MB — that is far too small to contain any real contacts or calendar data. It's essentially an empty file, which is exactly why no live iCloud data is shown in Outlook.

The older ******@icloud.com - Outlook1214.ost at 83 MB from 2024 is much larger and likely contains my actual iCloud contacts and calendar data — but Outlook is using the wrong file!

The current iCloud .ost file is only 16MB and appears empty. An older iCloud .ost file from 2024 is 83MB and likely contains the actual contacts and calendar data. Outlook appears to be using the wrong iCloud data file.

C:\Users\A\AppData\Local\Microsoft\Outlook

9 File(s) 4,254,375,936 bytes

0 Dir(s) 525,452,222,464 bytes free

Date Time Size File

1 06/04/26 10:48 AM 16,818,176 ******@hotmail.com.ost

2 06/04/26 10:40 AM 16,818,176 ******@icloud.com.ost

3 06/04/26 10:37 AM 16,818,176 ******@gmail.com(3).ost

4 06/04/26 10:37 AM 16,818,176 ******@gmail.com.ost

5 06/04/26 10:37 AM 16,818,176 ******@gmail.com.ost

6 06/04/26 10:37 AM 16,818,176 ******@gmail.com.ost

7 02/18/26 10:56 AM 2,501,976,064 ******@comcast.net - New Outlook.ost

8 02/05/24 9:18 AM 83,533,824 ******@icloud.com - Outlook1214.ost

9 02/05/24 9:18 AM 1,567,956,992 ******@comcast.net - Outlook1214.ost

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3 answers

Your answer

Answer 1

Classic Outlook - iCloud for Windows Setup

These instructions are for creating a new Outlook Profile and setting up the iCloud for Windows email, Calendar, and Contacts sync. Once the setup for this Profile has been completed, you can add additional (non-iCloud) accounts using the normal Outlook File -> [+ Add Account] process.

If you are adding an iCloud account to an existing Outlook profile:

Do steps #1 and #2
Start the normal Outlook add account process ( File -> [+ Add Account] )
Continue at Step #5 and ignore the steps that have you switching profiles.

1 -- Latest iCloud for Windows installed?

Download and install the latest iCloud for Windows from
https://apps.microsoft.com/detail/9pktq5699m62

2 -- iCloud connected and configured?

From your Desktop double-click the iCloud for Windows icon or use the Start Menu Search to find iCloud.
Open iCloud for Windows and ensure that you have a connection to your iCloud account,

3 -- Start Outlook with the /profiles switch

Close Outlook.
Windows Key + r (open the Windows Run Dialog).
Enter Outlook.exe /profiles in the Open box and press {Enter}

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4 -- Start the New Profile process

On the "Choose Profile" dialog click [Options >>]
Enable "Prompt for a profile to be used"
Click [New ...]
Profile Name: iCloud Only Test
Click [OK]

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5 -- Add your iCloud Account

Enter your iCloud email address and click [Connect]
Enter your iCloud account password and click [Connect]
If you have Two Factor Authentication on your iCloud Account you must use an Apple Generated App Password. See https://support.apple.com/en-us/102654
On the "Account successfully added" dialog click [Done]
On the Choose Profile dialog click [Close]

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6 -- Open Outlook and choose your iCloud Only Profile

Open Outlook
On the Choose Profile dialog select your iCloud Only Profile
Click [OK]
Outlook will open showing your iCloud emails

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7 -- Add a local iCloud PST and make it the Default

From Outlook do File -> Account Settings -> Account Setting
On the "Data files" tab click [Add ...]
On the "New Outlook Data File" dialog choose "Outlook data file (.pst) and click [OK]
Pick a location, name the PST file, and click [OK]
On the Data Files tab select the PST file and [Set as Default]
On the Mail Delivery Location dialog click [OK]
The PST file is now set as the Default.
Close the Account Settings dialog.

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8 -- Get iCloud to sync Calendar and Contacts to the Profile

Close Outlook.
Open the iCloud for Windows app
Toggle the "Calendars and Contacts" sync switch Off (if it is not already) and then back On again.
You should see the "Setup is complete" dialog

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9 -- Verify the iCloud Calendar and Contacts Sync

Open Outlook.
Choose the iCloud Only profile
Verify that you see "iCloud" Contacts and Calendar

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Answer 2

Hornblower409 16,350

@Drewskii said

duplicate and conflicting data files that are confusing Outlook and preventing iCloud from registering properly

No doubt. But before trying to untangle the mess, I would suggest that you test your Outlook iCloud connection using a clean slate.

1 -- Make sure you have the latest iCloud for Windows installed.
https://apps.microsoft.com/detail/9pktq5699m62

2 -- Run iCloud for Windows and ensure that you have a connection to your iCloud account and that sync Calendars and Contacts is enabled.

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3 -- Create a new Outlook Profile and add only your iCloud account.
https://support.microsoft.com/en-us/office/create-an-outlook-profile-f544c1ba-3352-4b3b-be0b-8d42a540459d

Does the new iCloud only profile sync correctly and pull down your iCloud Calendar and Contacts?

Drewskii 0 Reputation points

2026-06-06T12:58:39.39+00:00

Provide a detailed tutorial for creating a new Outlook profile to add only my iCloud account with step-by-step instructions for each screen encountered in the process

Answer 3

To restore working contacts/calendar sync and clean up the OST/PST situation, use the supported approach of rebuilding the OST and, if needed, exporting/importing data via PST. The specific older OST file cannot be “switched back in” as a live store; Outlook will always create and bind to a fresh OST for the account.

Follow these steps in classic Outlook for Windows:

Protect any local-only data
- If there are any contacts, calendar items, or folders that exist only on the PC (not in the cloud/mailbox), export them first:
  1. In Outlook, select File > Open & Export > Import/Export.
  2. Choose Export to a file > Next.
  3. Choose Outlook Data File (.pst) > Next.
  4. Select the relevant Contacts/Calendar folders (for each account as needed), enable Include subfolders, then Next.
  5. Choose a save location and file name, then Finish.
Rebuild the OST for the affected account(s)
- Close Outlook.
- Go to the Outlook data folder shown: C:\Users\A\AppData\Local\Microsoft\Outlook.
- For each problematic account (e.g., ******@icloud.com.ost, ******@hotmail.com.ost, ******@gmail.com.ost):
  1. Rename the current .ost file to .old (for example, ******@icloud.com.ost → ******@icloud.com.old).
  2. If Outlook reports “File is in use” when renaming, open Task Manager, go to Processes, and ensure Outlook.exe and Winword.exe are not running; end those processes if present.
- Do not try to force Outlook to use the older ******@icloud.com - Outlook1214.ost directly; instead, let Outlook create a new OST and re-sync from the server.
- Start Outlook again. It will display a “Preparing for first use” or similar message and recreate new OST files, then download mail, contacts, and calendar from the server. Wait for synchronization to complete.
Verify sync and contacts/calendar
- After the new OST is built and sync finishes, check:
  - Mail folders for each account.
  - Contacts/People view.
  - Calendar view.
- If contacts and calendar now appear and stay in sync with iCloud and other services, the old, conflicting OSTs were the cause.
If contacts/calendar are still missing
- If the data exists on the server (for example, visible in Outlook on the web or in the iCloud web interface), the rebuilt OST should eventually bring it down. If it does not:
  - Confirm that the account is configured as an Exchange/modern connection and that iCloud add-in or connector is correctly installed and enabled.
- If the only known copy of some items was local and was previously in an OST, use the PST exported in step 1:
  1. In Outlook, select File > Open & Export > Import/Export.
  2. Choose Import from another program or file > Outlook Data File (.pst).
  3. Select the PST exported earlier.
  4. Choose Do not import duplicates.
  5. Map Contacts and Calendar folders to the appropriate mailbox folders.
If synchronization issues persist
- For Outlook 2010 and later, if a specific folder still appears out of sync:
  1. Right-click the folder (for example, Contacts or Calendar) and select Properties.
  2. Select Clear Offline Items > OK.
  3. On the Send/Receive tab, select Update Folder.
- If this still does not resolve the issue after OST rebuild and folder resync, the problem is likely not the OST file itself; further investigation or contacting support is recommended.
Use Scanpst.exe only if the OST/PST is corrupt
- If Outlook reports that it cannot open a data file or that the file is corrupted, run the Inbox Repair tool (SCANPST.EXE) against the affected OST or PST to verify and repair structural errors.
- This tool checks and repairs the file structure but does not interact with server-side mailboxes.

Key points:

Outlook 2010 and later do not use Scanost; OST issues are resolved by resyncing folders or rebuilding the OST.
Old OSTs (such as Outlook1214.ost) are not meant to be reused as live stores; instead, Outlook should recreate a fresh OST and sync from the server.
Always export local-only data to PST before deleting or renaming OST files to avoid data loss.

References: