There are a number of mesh tools available with OGRE to help you manipulate your meshes.

For getting data out of modellers and into OGRE.

For converting meshes and skeletons to/from XML.

For upgrading binary meshes from one version of OGRE to another.

Exporters

Exporters are plugins to 3D modelling tools which write meshes and skeletal animation to file formats which OGRE can use for realtime rendering. The files the exporters write end in .mesh and .skeleton respectively.

Each exporter has to be written specifically for the modeller in question, although they all use a common set of facilities provided by the classes MeshSerializer and SkeletonSerializer. They also normally require you to own the modelling tool.

All the exporters here can be built from the source code, or you can download precompiled versions from the OGRE web site.

A Note About Modelling / Animation For OGRE

There are a few rules when creating an animated model for OGRE:

You must have no more than 4 weighted bone assignments per vertex. If you have more, OGRE will eliminate the lowest weighted assignments and re-normalise the other weights. This limit is imposed by hardware blending limitations.
All vertices must be assigned to at least one bone - assign static vertices to the root bone.
At the very least each bone must have a keyframe at the beginning and end of the animation.

If you’re creating non-animated meshes, then you do not need to be concerned with the above.

Full documentation for each exporter is provided along with the exporter itself, and there is a list of the currently supported modelling tools in the OGRE Wiki at http://www.ogre3d.org/tikiwiki/tiki-index.php?page=OGRE+Exporters&structure=Tools.

XMLConverter

The OgreXMLConverter tool can converter binary .mesh and .skeleton files to XML and back again - this is a very useful tool for debugging the contents of meshes, or for exchanging mesh data easily - many of the modeller mesh exporters export to XML because it is simpler to do, and OgreXMLConverter can then produce a binary from it. Other than simplicity, the other advantage is that OgreXMLConverter can generate additional information for the mesh, like bounding regions and level-of-detail reduction.

Syntax:

Usage: OgreXMLConverter sourcefile [destfile]
sourcefile = name of file to convert
destfile   = optional name of file to write to. If you don't
             specify this OGRE works it out through the extension
             and the XML contents if the source is XML. For example
             test.mesh becomes test.xml, test.xml becomes test.mesh
             if the XML document root is <mesh> etc.

When converting XML to .mesh, you will be prompted to (re)generate level-of-detail(LOD) information for the mesh - you can choose to skip this part if you wish, but doing it will allow you to make your mesh reduce in detail automatically when it is loaded into the engine. The engine uses a complex algorithm to determine the best parts of the mesh to reduce in detail depending on many factors such as the curvature of the surface, the edges of the mesh and seams at the edges of textures and smoothing groups - taking advantage of it is advised to make your meshes more scalable in real scenes.

MeshUpgrader

This tool is provided to allow you to upgrade your meshes when the binary format changes - sometimes we alter it to add new features and as such you need to keep your own assets up to date. This tools has a very simple syntax:

OgreMeshUpgrader <oldmesh> <newmesh>

The OGRE release notes will notify you when this is necessary with a new release.