What is the OLB file format?
A .olb file is a COM/OLE type library - a binary file that describes the interfaces, coclasses, enumerations, and method signatures exposed by a COM component. It is functionally identical to a .tlb file; the .olb extension is the convention used by Microsoft Office applications (Word, Excel, Outlook) for their type libraries.
Type libraries allow development tools and scripting environments to enumerate a COM object's capabilities at design time. In the Visual Basic for Applications (VBA) IDE, adding a reference to an .olb file via Tools → References gives IntelliSense access to the complete Office object model.
The binary format begins with either the MSFT magic bytes (4D 53 46 54) for modern type libraries (Windows 2000 and later) or the older SLTG signature (53 4C 54 47) used by Visual Basic 5/6-era components. The MSFT format is documented in Microsoft's open specifications under MS-OAUT.
.olb files are registered in the Windows Registry under HKEY_CLASSES_ROOT\TypeLib with their associated GUID and version. Users and developers should not move or modify these files manually - doing so breaks the Registry entries and may prevent Office applications from loading correctly.
Modern successors include .winmd (Windows Metadata) for WinRT/UWP components and inline metadata in .NET assemblies, but .olb files remain in active use throughout the Office COM automation ecosystem.
Security & safety
RISK: LOWLegitimate OLB files are read-only type metadata and cannot execute code on their own. However, many websites offering "fix missing .olb" downloads distribute adware or PUPs. Never download individual OLB files from third-party sites - always reinstall the original application (Microsoft Office, etc.) to restore them.
Format details
in a nutshell- Cadence OrCAD / PSpice Symbol Library - Cadence electronics EDA tools (OrCAD PCB Designer, PSpice) use .olb for component symbol libraries - an entirely different proprietary format unrelated to COM/OLE.
Programs that open OLB files
Windows3 apps
Microsoft Visual Studio
Freemium Project → Add Reference → COM tab - loads OLB/TLB to generate interop wrappers. 
Microsoft Office (built-in, via VBA IDE)
Paid In VBA IDE: Tools → References → browse to the OLB file to register it for IntelliSense.
OLE/COM Object Viewer (oleview.exe)
Free File → View TypeLib → open the .olb file to browse interfaces and methods in human-readable form. Technical details
deep spec| Magic bytes (modern) | MSFT (4D 53 46 54) at offset 0 - MSFT format (Windows 2000+) |
| Magic bytes (legacy) | SLTG (53 4C 54 47) at offset 0 - legacy format used by VB5/6-era components |
| Format specification | MS-OAUT (Microsoft Open Specification for OLE Automation type libraries) |
| Byte order | Little-endian |
| Container | MSFT type library binary: header + segment directory + type info blocks + name/string tables |
| Type info content | COM interfaces, coclasses, enumerations, structs, and modules with full method signatures |
| Registry registration | Registered under HKEY_CLASSES_ROOT\TypeLib with GUID and version |
| Typical file size | 50 KB - 5 MB depending on the number of COM interfaces documented |
| Functional equivalence | Identical to .tlb; .olb is the Office application convention, .tlb is the generic convention |
| VBA integration | Referenced via VBA IDE Tools → References to enable IntelliSense for COM object models |
| Modern successor | .winmd (Windows Metadata) for WinRT/UWP; .NET assemblies carry type metadata inline |
| Associated OS | Windows only |
| Released | 1993 with OLE 2.0 / COM (Windows NT 3.1 era) |
| Latest version | N/A - format stable since Windows 2000; superseded by .winmd in WinRT |
| Specification | learn.microsoft.com |
Community Q&A
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