ost2pst turns an orphaned Outlook .ost file into a .pst file that Outlook can open. Orphaned means there is no Exchange account, no mail profile and no server left. There is only the file. That single case is the one Outlook itself will not handle: Import/Export, AutoArchive, drag-and-drop and VBA macros all need the OST mounted in a working profile first, and an orphaned file cannot be mounted.

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It is not a hosted tool

Every other tool on the public tools host is a path that runs on the box. This one is a Windows program you copy to the machine that holds the file.
The overview page says a second tool would arrive as a path under public-tools. That rule was written for tools the box runs. ost2pst does not run on the box at all, so it takes a page and a download instead of a path and a service.

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Windows will try to block it on first run

The executable is not code-signed, so SmartScreen shows “Windows protected your PC” the first time it runs. This is what Windows does with any unsigned executable. It is not a sign that anything is wrong with the file.
1

Click More info

The dialog hides the run button until you do.
2

Click Run anyway

The program starts normally after this. The warning does not return.
Some antivirus products also quarantine it. A single-file .NET bundle unpacks its own runtime at startup, and that behaviour resembles self-extracting malware closely enough to trip heuristics. If it is quarantined, the fix is an exclusion for this one file.Tell whoever is running it about both of these before they run it. Told afterwards, a security warning reads as confirmation that the file is unsafe.

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Licence status is unresolved, and that is why the repository is private

Do not redistribute this tool publicly, and do not make the repository public, until the licence question below is settled.
The chain of origin has a gap in it: Niv2023/ost2pst ships no LICENSE file. Under copyright default that means all rights reserved: no permission to copy, modify or redistribute was ever granted. The fork adds an MIT file, but a fork cannot grant a licence the upstream never gave, so that file records the fork author’s intent rather than settling the question. NOTICE.md in the tool directory carries the full chain and the local changes made on top of it. Read it before any decision about publishing.

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Local changes

The fork is used as-is apart from a --reset-password CLI flag added on top of it. Everything else is upstream behaviour.