Blender? I know how on blender2.77. And some OGRE plugin I can't remember how to install. Might still be tutorials on the internet.
"Import" the .obj by looking under file, import, wavefront(.obj).
Then i click on it, select "object mode" on the little selector, for me it's at the bottm of the screen. Has to be object mode not edit mode.
Then export as OGRE mesh. The OGRE plugin spits out a material file for it.
You might have to drag-drop the exported mesh file into the command line tool which will give you a new mesh file that the game can read.
Here's the ones I use just in case you need them.
http://www.mediafire.com/file/i8ccmilduap1cc8/OgreCommandLineTools_1.7.2.zip/file