OBBoxRenderable* mOBBoxRenderable = new OBBoxRenderable();
...
mYourEntity = mSceneMgr->createEntity("test", "test.mesh");
mSceneMgr->getRootSceneNode()->createChildSceneNode("test")->attachObject(mYourEntity );
mSceneMgr->getSceneNode("test")->setPosition(25.0f, 0.0f, 25.0f);
mOBBoxRenderable->setupVertices(mYourEntity->getBoundingBox());
mSceneMgr->getSceneNode("test")->attachObject(mOBBoxRenderable);
...
delete mOBBoxRenderable; don't forget to delete it when you clean up your resources.

#ifndef OBBOXRENDERABLE_H_
#define OBBOXRENDERABLE_H_

#include <OgreSimpleRenderable.h>
#include <OgreAxisAlignedBox.h>

private class OBBoxRenderable : public Ogre::SimpleRenderable
{
    Ogre::VertexData vertexes;
    public:
        OBBoxRenderable();
        void setupVertices(const Ogre::AxisAlignedBox& aab);
        virtual ~OBBoxRenderable();
        Ogre::Real getSquaredViewDepth(const Ogre::Camera*)const;
        Ogre::Real getBoundingRadius()const;
        virtual void getWorldTransforms (Ogre::Matrix4 *xform)const;
};

#endif /*OBBOXRENDERABLE_H_*/

#include "Ogre.h"

using namespace Ogre;

#include "OBBoxRenderable.h"

OBBoxRenderable::OBBoxRenderable()
{
    mRenderOp.vertexData = new Ogre::VertexData();
    mRenderOp.indexData = 0;
    mRenderOp.vertexData->vertexCount = 24;
    mRenderOp.vertexData->vertexStart = 0;
    mRenderOp.operationType = Ogre::RenderOperation::OT_LINE_LIST;
    mRenderOp.useIndexes = false;
    Ogre::VertexDeclaration* decl = mRenderOp.vertexData->vertexDeclaration;
    Ogre::VertexBufferBinding* bind = mRenderOp.vertexData->vertexBufferBinding;
    decl->addElement(0, 0, Ogre::VET_FLOAT3, Ogre::VES_POSITION);
    Ogre::HardwareVertexBufferSharedPtr vbuf = Ogre::HardwareBufferManager::getSingleton().createVertexBuffer(
        decl->getVertexSize(0), mRenderOp.vertexData->vertexCount, Ogre::HardwareBuffer::HBU_STATIC_WRITE_ONLY);
    // Bind buffer
    bind->setBinding(0, vbuf);
    // setup material
    Ogre::MaterialPtr matptrOBBoxManualMaterial = Ogre::MaterialManager::getSingleton().create("OBBoxManualMaterial", "General");
    matptrOBBoxManualMaterial->setReceiveShadows(false);
    matptrOBBoxManualMaterial->getTechnique(0)->setLightingEnabled(true);
    matptrOBBoxManualMaterial->getTechnique(0)->getPass(0)->setDiffuse(0, 1, 0, 0);
    matptrOBBoxManualMaterial->getTechnique(0)->getPass(0)->setAmbient(0, 1, 0);
    matptrOBBoxManualMaterial->getTechnique(0)->getPass(0)->setSelfIllumination(0, 1, 0);
    this->setCastShadows(false);
    //this->setQueryFlags(QF_UNKNOWN); // set a query flag to exlude from queries (if necessary).
    this->setMaterial("OBBoxManualMaterial");
}
    
OBBoxRenderable::~OBBoxRenderable()
{
    delete mRenderOp.vertexData;
}
    
void OBBoxRenderable::setupVertices(const Ogre::AxisAlignedBox& aab)
{
    Ogre::Vector3 vmax = aab.getMaximum();
    Ogre::Vector3 vmin = aab.getMinimum();
    Ogre::Real maxdistground = vmax.y;
    Ogre::Real mindistground = vmin.y;
    Ogre::Real maxx = vmax.x;
    Ogre::Real maxy = vmax.y;
    Ogre::Real maxz = vmax.z;
    Ogre::Real minx = vmin.x;
    Ogre::Real miny = vmin.y;
    Ogre::Real minz = vmin.z;
    // fill in the Vertex buffer: 12 lines with 2 endpoints each make up a box
    Ogre::HardwareVertexBufferSharedPtr vbuf = mRenderOp.vertexData->vertexBufferBinding->getBuffer(0);
    float* pPos = static_cast<float*>(vbuf->lock(Ogre::HardwareBuffer::HBL_DISCARD));
    // line 0
    *pPos++ = minx;
    *pPos++ = miny;
    *pPos++ = minz;
    *pPos++ = maxx;
    *pPos++ = miny;
    *pPos++ = minz;
    // line 1
    *pPos++ = minx;
    *pPos++ = miny;
    *pPos++ = minz;
    *pPos++ = minx;
    *pPos++ = miny;
    *pPos++ = maxz;
    // line 2
    *pPos++ = minx;
    *pPos++ = miny;
    *pPos++ = minz;
    *pPos++ = minx;
    *pPos++ = maxy;
    *pPos++ = minz;
    // line 3
    *pPos++ = minx;
    *pPos++ = maxy;
    *pPos++ = minz;
    *pPos++ = minx;
    *pPos++ = maxy;
    *pPos++ = maxz;
    // line 4
    *pPos++ = minx;
    *pPos++ = maxy;
    *pPos++ = minz;
    *pPos++ = maxx;
    *pPos++ = maxy;
    *pPos++ = minz;
    // line 5
    *pPos++ = maxx;
    *pPos++ = miny;
    *pPos++ = minz;
    *pPos++ = maxx;
    *pPos++ = miny;
    *pPos++ = maxz;
    // line 6
    *pPos++ = maxx;
    *pPos++ = miny;
    *pPos++ = minz;
    *pPos++ = maxx;
    *pPos++ = maxy;
    *pPos++ = minz;
    // line 7
    *pPos++ = minx;
    *pPos++ = maxy;
    *pPos++ = maxz;
    *pPos++ = maxx;
    *pPos++ = maxy;
    *pPos++ = maxz;
    // line 8
    *pPos++ = minx;
    *pPos++ = maxy;
    *pPos++ = maxz;
    *pPos++ = minx;
    *pPos++ = miny;
    *pPos++ = maxz;
    // line 9
    *pPos++ = maxx;
    *pPos++ = maxy;
    *pPos++ = minz;
    *pPos++ = maxx;
    *pPos++ = maxy;
    *pPos++ = maxz;
    // line 10
    *pPos++ = maxx;
    *pPos++ = miny;
    *pPos++ = maxz;
    *pPos++ = maxx;
    *pPos++ = maxy;
    *pPos++ = maxz;
    // line 11
    *pPos++ = minx;
    *pPos++ = miny;
    *pPos++ = maxz;
    *pPos++ = maxx;
    *pPos++ = miny;
    *pPos++ = maxz;
    vbuf->unlock();
    // setup the bounding box of this SimpleRenderable
    setBoundingBox(aab);
}
    
Ogre::Real OBBoxRenderable::getSquaredViewDepth(const Ogre::Camera* cam)const
{
    Ogre::Vector3 min, max, mid, dist;
    min = mBox.getMinimum();
    max = mBox.getMaximum();
    mid = ((max - min) * 0.5) + min;
    dist = cam->getDerivedPosition() - mid;
    return dist.squaredLength();
}
    
Ogre::Real OBBoxRenderable::getBoundingRadius()const
{
    return 0;
}
    
void OBBoxRenderable::getWorldTransforms (Ogre::Matrix4 *xform)const
{
    SimpleRenderable::getWorldTransforms (xform);
}