CMO File

What are CMO files and how to open them

Are you having problems opening a CMO file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your CMO files.

What is a CMO file?

CMO files have multiple uses, and 3DVIA Virtools Behavioral Object is one of them. Read more about the other uses further down the page.

3DVIA Virtools Behavioral Object

The 3DVIA Virtools Behavioral Object format is a composition file used by the Virtools development environment to build interactive 3D scenes. It is an editable format in which authors assemble objects such as 3D models, cameras, textures, materials, lights, sprites, and sounds, then attach behaviors - scripts, behavior graphs, or building blocks - to enable interactivity and define how the scene responds to user input or environmental factors. The format serves as the author source version of a virtual environment, capturing all elements, logic, and structure needed for development and editing.

These files are produced by Virtools Dev (part of the 3DVIA suite from Dassault Systèmes). Between authoring work, placing elements, setting up scenes and "Places," configuring behavior links and flow between behavior building blocks, everything is saved into this format. Once a composition is complete, it can alternatively be exported to a view-only file (VMO) for distribution or embedding in web-player contexts, whereas this format remains fully editable within Virtools Dev.

CMO files begin with a specific header: at the very start, the ASCII sequence NemoFi (that is, bytes 4E 65 6D 6F 20 46 69) appears, which helps tools identify this format.

How to open CMO files

Important: Different programs may use files with the CMO file extension for different purposes, so unless you are sure which format your CMO file is, you may need to try a few different programs.

While we have not verified the apps ourselves yet, our users have suggested six different CMO openers which you will find listed below.

Last updated: August 31, 2026

All known file formats using extension .CMO

While 3DVIA Virtools Behavioral Object is a popular type of CMO-file, we know of 2 different uses of the .CMO file extension. Different software may use files with the same extension for different types of data.

OCaml Bytecode (object)

OCaml Bytecode (object) files store compiled versions of individual OCaml module implementations in bytecode form. These files are created from OCaml source files by the ocamlc command using the -c (compile only) option. During compilation, source code (.ml files) is turned into a .cmo object which pairs with a compiled interface (.cmi) that specifies the module's public API.

Such files are intended for use by the OCaml bytecode linker and interpreter (ocamlrun). Multiple .cmo files (often along with .cma libraries) can be linked together to form a bytecode executable. Bytecode object files get combined in a specified order, because module initialization at runtime follows that order.

Internally, a .cmo file begins with a fixed magic header, followed by a pointer to its compilation unit descriptor, then a block of relocatable bytecode, optional debugging metadata, and finally a marshaled compilation unit descriptor. The magic header starts with the ASCII string Caml1999O.

Various apps that use files with this extension

These apps are known to open certain types of CMO files. Remember, different programs may use CMO files for different purposes, so you may need to try out a few of them to be able to open your specific file.

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