Viewport::SetOrientation must be called after a frame is rendered to take effect. For example to change to Viewport::OR_PORTRAIT:
Q: I get error "OGRE EXCEPTION(7:InternalErrorException): Cannot create GL vertex buffer in GLESHardwareVertexBuffer::GLESHardwareVertexBuffer" when I try to load meshes, how can I get rid of this ?
It seems that the GLES render system can't create buffers just before rendering one frame.
Verify that your load() methods are not in a method called just before the rendering of a frame - methods like frameStarted().
On the other hand, you can load your meshes just after the rendering - methods like frameEnded().
Out of the rendering loop, GLES seems to not have that problem.